Every car enthusiast in Australia has asked the same question: why does my car get water spots after I wash it? The answer is in your tap water. Specifically, it is the Total Dissolved Solids (TDS) in your water that leave mineral deposits on your paint, glass and trim as the water evaporates.
The problem is that Australian tap water quality varies enormously from city to city, and even from suburb to suburb within the same city. Melbourne's mountain catchments deliver some of the purest tap water in the world at 30 to 70 ppm, while parts of Perth's northern suburbs sit above 580 ppm. That is a 10x difference, and it means the right setup for your area depends on your local water. The good news: a DI tank produces 0 ppm pure water regardless of what goes in, so every Spot Free Rinse customer gets the same perfect, spot-free result.
We built this guide to give you the real numbers for your area, sourced from official water authority reports, lab analyses and verified meter readings across all eight capital cities and dozens of regional centres. Use the lookup tool below to find your local TDS, understand what it means for water spots and resin life, and see which DI tank size suits your water.
Find Your Local TDS
Enter your suburb or postcode below to see your local tap water TDS range. Your result includes the water authority source, what it means for water spots, and a recommendation for your area.
We do not have data for that location yet. You can still use a DI tank with any water. Order a tank and use the free TDS meter included with every order to test your own water.
Browse DI Tanks →What Is TDS and Why Does It Matter for Car Washing?
TDS stands for Total Dissolved Solids. It is a measurement (in parts per million, or ppm) of everything dissolved in your water: calcium, magnesium, sodium, chloride, bicarbonate, sulfate, silica and more. When you rinse your car with tap water and it evaporates, all those dissolved minerals are left behind as white spots and streaks on your paint, glass and trim.
The higher your TDS, the worse the water spots. At 50 ppm (Melbourne), the spots are barely visible. At 250 ppm (Brisbane south), they are obvious. At 580 ppm (Perth northern suburbs), they are severe and can etch into clear coat if left in the sun.
A DI (deionised) water tank removes these dissolved minerals by passing your tap water through ion exchange resin. The output is 0 ppm pure water that evaporates cleanly with zero spots. No drying needed. Wash. Rinse. Pack up.
TDS vs Hardness: They Are Not the Same Thing
Most Australian water authorities publish hardness, not TDS. Hardness measures only calcium and magnesium (the minerals that cause limescale). TDS measures everything dissolved in the water. TDS is always higher than hardness, sometimes two to three times higher. For example, Brisbane's published hardness is ~115 mg/L, but actual TDS meter readings in Brisbane south are 180 to 300 ppm. If you see a figure labelled "hardness," do not assume that is your TDS. Use a TDS meter for the true reading.
How TDS Affects DI Resin Life
DI resin exchanges its clean hydrogen and hydroxide ions for the dissolved minerals in your water. The more minerals your water contains, the harder the resin works for you on every rinse. Your local TDS determines how many litres of pure water you get from a tank, which is why choosing the right tank size for your area makes all the difference.
As a rough guide for a 10-litre DI tank at 30 litres per final rinse:
- 50 ppm (Melbourne): ~5,100 to 5,800 litres, or 170 to 194 rinses
- 150 ppm (Sydney): ~1,700 to 1,933 litres, or 57 to 65 rinses
- 250 ppm (Brisbane south): ~1,020 to 1,160 litres, or 34 to 39 rinses
- 400 ppm (Adelaide, Perth south): ~638 to 725 litres, or 21 to 24 rinses
- 580 ppm (Perth north): ~440 to 500 litres, or 15 to 17 rinses
If your TDS is above 300, stepping up to a 20L or 25L tank is the smart move. More resin means more washes per fill and a lower cost per wash. If your TDS is above 500, pairing your DI tank with an RO (reverse osmosis) pre-filter is a game changer. The RO knocks out 90 to 95% of minerals before the water even touches the resin, giving you dramatically more washes from every fill.
Use the resin life calculator to get a precise estimate for your water and your vehicle.
Australian Tap Water Quality: A Quick Overview
Australia's tap water TDS ranges from below 50 ppm in the best mountain catchments to over 600 ppm in some regional groundwater supplies. The NHMRC Australian Drinking Water Guidelines set an aesthetic guideline of 600 ppm for TDS. Most capital cities are well below that, but some Perth suburbs and South Australian towns approach it.
The short version:
- Softest water (best for car washing): Hobart (49 ppm), Melbourne (50 ppm), Central Coast NSW (46 ppm), Canberra (72 ppm)
- Moderate: Sydney (100 to 140 ppm), Gold Coast (90 ppm), Brisbane north (125 ppm), Sunshine Coast (110 ppm), Darwin (140 ppm)
- Elevated: Brisbane south/Ipswich (250 ppm), Newcastle (170 ppm), Geelong (200 ppm), Ballarat (180 ppm)
- High: Adelaide (380 ppm), Perth south (400 ppm), Perth central (480 ppm), Kalgoorlie (469 ppm), Mildura (300 ppm), Alice Springs (400 ppm)
- Very high: Perth northern suburbs (584 ppm), Port Lincoln (560 ppm), Geraldton (858 ppm)
Why so much variation? It comes down to the water source. Protected mountain catchments (Melbourne, Hobart, Canberra) produce naturally pure water. Limestone aquifers and the Murray River carry more minerals. Groundwater from the Gnangara Mound in Perth's north is filtered through limestone, picking up calcium and magnesium. Desalinated seawater (used in Perth and Adelaide) starts near zero but is remineralised before supply.
Bore Water, Tank Water and Dam Water
If you are not on mains supply, your TDS could be very different from the figures above. Domestic bore water in Australia typically ranges from 300 to 1,500 ppm, with some Western Australian bores exceeding 5,000 ppm. Rainwater tank water is usually very low (10 to 30 ppm). Dam water varies with catchment geology.
Whatever your source, the free TDS meter included with every Spot Free Rinse tank will tell you exactly what you are working with.
How to Test Your Water
A TDS meter is a small pen-sized device that gives you an instant reading in ppm. Dip it in a glass of your tap water and read the number. Every Spot Free Rinse DI tank includes a free TDS meter so you can:
- Test your tap water to know your input TDS
- Test the output from your DI tank to confirm it is producing 0 ppm pure water
- Monitor when the resin is exhausted (output TDS starts rising above 0)
Readings vary by suburb, season, and even time of day. Water authorities blend sources depending on demand, so your TDS might be lower in winter (more reservoir water) and higher in summer (more groundwater or Murray River water). Test a few times across the year to understand your range.
Choosing the Right DI Tank for Your Water
Your TDS determines the ideal tank size. In low-TDS areas (under 100 ppm), a 10L tank is all you need and will last dozens of washes. In higher-TDS areas (300+ ppm), a 20L or 25L tank is the better match. You get more washes per fill, a lower cost per wash, and the same perfectly spot-free results every time.
Ready to see exactly how many washes you will get? Visit the product page, enter your TDS into the calculator, and see the numbers for each tank size. Every tank ships with a free TDS meter so you can confirm your reading on day one.
Frequently Asked Questions
What is TDS in water?
TDS (Total Dissolved Solids) is a measurement of all the dissolved minerals, salts and metals in your water, expressed in parts per million (ppm) or milligrams per litre (mg/L). These are the minerals that cause water spots on cars, glass and surfaces when water evaporates.
What TDS level causes water spots on cars?
Any TDS above 0 can leave spots, but they become noticeable above about 50 ppm and problematic above 150 ppm. On dark or metallic paint, spots are visible at lower TDS levels than on white or silver paint. For truly spot-free results, you need 0 ppm water from a DI tank.
Is Australian tap water hard or soft?
It depends entirely on where you live. Melbourne, Hobart, and Canberra have very soft water (under 80 ppm TDS). Sydney is soft to moderate (80 to 150 ppm). Brisbane varies dramatically, from 90 ppm in the north to 300 ppm in the south. Perth and Adelaide have hard water, with some Perth suburbs exceeding 580 ppm.
Which Australian city has the hardest water?
Among capital cities, Perth has the hardest water overall, with northern suburbs (Yanchep, Butler, Joondalup) typically around 580 ppm TDS due to Gnangara Mound limestone groundwater. Adelaide is second, averaging around 380 ppm with seasonal peaks up to 480 ppm when Murray River contribution is high. Among all Australian towns, Geraldton (WA) at 858 ppm is one of the highest on reticulated supply.
Which Australian city has the softest water?
Hobart has the softest capital city water at around 49 ppm TDS, closely followed by Melbourne at around 50 ppm. Both benefit from protected mountain/rainforest catchments with minimal mineral contact. The Central Coast of NSW is also exceptionally soft at around 46 ppm.
Does bore water cause more water spots than tap water?
Usually yes, and often significantly more. Australian domestic bore water typically ranges from 300 to 1,500 ppm TDS, and some Western Australian bores exceed 5,000 ppm. Even "good" bore water at 300 ppm will leave worse spots than Sydney tap water at 130 ppm. If you wash with bore water, a DI tank is essential.
How do I test my water TDS?
Use a TDS meter (also called a TDS pen). Fill a glass with cold tap water, dip the meter in, and read the number in ppm. Every Spot Free Rinse DI tank includes a free TDS meter. You can also buy standalone TDS meters from hardware stores or online for around $10 to $20.
What is the difference between TDS and water hardness?
Hardness measures only calcium and magnesium (the minerals that cause limescale in kettles and pipes). TDS measures everything dissolved in the water, including calcium, magnesium, sodium, chloride, bicarbonate, sulfate, silica and more. TDS is always higher than hardness. For water spots, TDS is the relevant measurement because all dissolved minerals leave spots, not just calcium and magnesium.
Does high TDS damage car paint?
High TDS water spots do not typically damage paint directly, but they can etch into clear coat if left to bake in the sun. The minerals in hard water are slightly alkaline and can leave permanent marks on clear coat, especially at TDS levels above 400 ppm. Prevention (rinsing with DI water) is far easier than correction (polishing out water spot etching).
How does TDS affect DI resin life?
Higher TDS means the resin works harder for you on each rinse. At 50 ppm (Melbourne), a 10L DI tank lasts roughly 170 to 194 rinses. At 250 ppm (Brisbane south), the same tank gives 34 to 39 rinses. At 500 ppm, a 10L gives around 17 to 19 rinses, which is why stepping up to a 20L or 25L tank makes sense in those areas. The result is always the same: perfectly spot-free water at 0 ppm. Knowing your TDS just helps you pick the right tank size so you get the best value.
What TDS is too high for spot-free rinsing with DI?
A DI tank produces spot-free 0 ppm water at any TDS level. Above 500 ppm, the best setup is to pair your DI tank with an RO (reverse osmosis) pre-filter. The RO removes 90 to 95% of the minerals first, and the DI polishes the remaining 5 to 10% to a perfect 0 ppm. This combination gives you outstanding resin life and the lowest possible cost per wash, even in the hardest water areas in Australia.
Does water quality change by season in Australia?
Yes, particularly in Adelaide and other cities that blend reservoir water with Murray River water. In summer and drought, a higher proportion of Murray River water is used, which has more dissolved minerals. Adelaide's TDS can swing from around 200 ppm in winter to 480 ppm in summer. Brisbane, Sydney and Perth also see seasonal variation, though less dramatic. Testing your water a few times across the year gives you a reliable range.
Why is Perth water so hard?
Perth's water comes from three sources: groundwater from the Gnangara and Jandakot aquifers (filtered through limestone, so high in calcium and magnesium), surface water from dams (moderate TDS), and desalinated seawater (very low TDS, remineralised). The northern suburbs get more Gnangara groundwater, so they have the highest TDS. Southern suburbs near the Kwinana desalination plant get more desalinated water and have lower TDS.
Why does Brisbane south have higher TDS than Brisbane north?
Brisbane south (including suburbs like Pallara, Sunnybank, Annerley and West End) is supplied by the Mt Crosby Water Treatment Plant, which draws from the Brisbane River via Wivenhoe Dam. The Wivenhoe catchment includes basalt and alluvial geology that releases more minerals. Brisbane north (North Lakes, Redcliffe, Kallangur) is supplied by the North Pine Water Treatment Plant, which draws from North Pine Dam in a sandstone catchment that releases far fewer minerals. The result: TDS of 180 to 300 ppm in the south versus 90 to 160 ppm in the north.
Is Melbourne water good for car washing?
Melbourne has some of the best tap water in Australia for car washing, at around 30 to 70 ppm TDS. You will get minimal water spots from Melbourne tap water compared to Perth or Adelaide. That said, even at 50 ppm, some spots will form on dark paint. A DI tank in Melbourne will last an exceptionally long time between resin changes, making it a very cost-effective solution.
Frequently Asked Questions
Does my local water quality affect how fast DI resin runs out?
Yes — this is the single biggest factor in resin life. Higher TDS water exhausts resin faster because there are more dissolved minerals to remove. A 10L tank on 50 ppm Canberra water will last roughly 3 times longer than the same tank on 150 ppm Sydney water, and up to 6 times longer than on 300 ppm Perth water.
Is rainwater already spot-free?
Usually, yes. Clean rainwater typically has a TDS of 5–20 ppm, which is low enough that most people will not see visible water spots. However, rainwater collected from a roof can pick up dust, pollen, and debris that increase TDS. If you use tank water, test it with a TDS meter — if it reads under 10 ppm, you may not need a DI system at all.
Should I use a water softener before my DI tank?
In areas with very hard water (250+ ppm), adding a water softener before the DI tank can extend resin life by removing the bulk of calcium and magnesium first. The DI resin then only needs to polish the remaining sodium and trace minerals to reach 0 ppm. This two-stage approach is cost-effective in Perth, Adelaide, and regional WA where TDS is highest.
Why does Perth water cause more water spots than Melbourne water?
Perth metropolitan tap water typically measures 200–350 ppm TDS, while Melbourne water is only 40–80 ppm. Higher TDS means more dissolved minerals in every drop. When that water evaporates on your car, Perth water leaves roughly 4–5 times more mineral residue per drop than Melbourne water — resulting in much more visible spotting.
