How Often Should Pool Water Be Changed? The Drain Triggers

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Pool water does not need changing on a calendar; replace part of it when a reading passes a ceiling only dilution fixes: stabiliser above 50 mg/L, calcium hardness above 400 mg/L in concrete or 200 in fibreglass, or dissolved solids above 1,500 mg/L in a non-salt pool. A 25 kL partial drain costs about $53–$146 in water at 2026–27 tariffs.

The short answer

No health department sets an interval. NSW and Queensland Health say high dissolved solids are managed by exchanging water; the trigger is a reading, not a date.
The three ceilings: stabiliser 50 mg/L (all four state documents we read); calcium 400 mg/L masonry, 200 fibreglass (NSW Health 2026); dissolved solids 1,500 mg/L in a non-salt pool (NSW Health 2026).
Count the tap water: share to replace = (reading − target) ÷ (reading − tap). Dissolved solids 2,500 → 1,500 in 50,000 L takes 21.1 kL on Sydney water (TDS 128) but 24.7 kL in South Perth (477).
What 25 kL costs, 2026–27: $85.25 in Sydney; $52.70–$131.40 in Perth by tier; $95.78–$146.01 in Melbourne (water and sewer usage); $63.00–$97.45 in Adelaide.
Partial, not full: the worked triggers in the table need 29–63% of the water replaced, not all of it; emptying a shell carries its own risk.

The drain-trigger table: readings that only replacing water fixes, the ceiling and its source (public-pool figures labelled), the share to replace and a worked 50,000 L example (tap-water figures from Sydney Water, 2025–26, and Water Corporation, 2024–25; read 20 October 2026)
Reading	Ceiling	Source	Share to replace	Worked, 50,000 L pool
Stabiliser (cyanuric acid)	Above 50 mg/L	NSW Health 2026 and Queensland Health (public, max 50); SA Health and HealthyWA (home, 30–50)	1 − target ÷ reading (tap water has none)	80 → 40 mg/L: 50.0%, 25.0 kL
Calcium hardness	Above 400 mg/L concrete; above 200 mg/L fibreglass	NSW Health 2026 Appendix E; SA Health home range tops out at 400	(reading − target) ÷ (reading − tap)	Fibreglass 300 → 200 on Sydney water (Potts Hill calcium hardness 31–34): 37.6%, 18.8 kL
Total dissolved solids, chlorine (non-salt) pool	Above 1,500 mg/L	NSW Health 2026: above 1,500 can indicate lack of dilution (public pools)	(reading − target) ÷ (reading − tap)	2,500 → 1,500 on Sydney water (TDS 128): 42.2%, 21.1 kL; South Perth water (477): 49.4%, 24.7 kL
Salt, salt chlorinator pool	Above the chlorinator manual’s maximum	Maker manuals; no health department sets a salt-pool TDS ceiling	(reading − target) ÷ (reading − tap)	7,000 → 5,000 ppm on Sydney water: 29.1%, 14.6 kL
Combined chlorine that shocking does not clear	Above 1.0 mg/L after a breakpoint shock	NSW Health 2026 (public): exchange a proportion with drinking water; limited where the supply is chloraminated	Not published	Retest after each partial exchange
Diarrhoea incident, stabilised pool	Stabiliser above 15 mg/L before hyperchlorination	NSW Health 2026 (public): may involve partially draining	1 − 15 ÷ reading	From 40 mg/L: 62.5%, 31.3 kL

How often should I change my pool water? When a reading passes a ceiling

There is no published interval, and the number of years people quote is a rule of thumb, not a standard. What the health guidance gives instead is a set of readings that chemicals cannot bring down. NSW Health’s 2026 guidelines: high dissolved solids are generally managed by exchanging water with fresh make-up water; calcium hardness is reduced by exchanging water with lower-calcium water; stabiliser may need partial draining. Queensland Health’s troubleshooting table says the same for a salty taste: dissolved solids too high, dilute with mains water.

The ceiling rule: change water when, and only as much as, a reading in the table is over its ceiling; if none is, do not drain. How fast you get there depends on the chlorine and the tap water. On liquid chlorine or a salt chlorinator, stabiliser leaves only with water; on dichlor it can climb from 30 to 50 mg/L in about 2½ to 6 weeks (how to raise pool chlorine works it). Evaporation leaves calcium behind with every top-up, faster on hard tap water (how to lower pool hardness).

Worked cost. Replacing half of a 50,000 L pool to take stabiliser from 80 to 40 mg/L is 25 kL. At 2026–27 prices that is $85.25 on Sydney Water’s flat $3.41/kL; $52.70 on Perth’s tier 1 and $131.40 on tier 3; $95.78–$146.01 on Yarra Valley Water’s three steps; $63.00–$97.45 on SA Water’s tiers.

Utility and tariff, 2026–27	Price	14.6 kL	25 kL	50 kL (whole pool)
Sydney Water, flat	$3.41/kL	$49.79	$85.25	$170.50
Perth, Water Corporation (tier 1 to tier 3)	$2.108–$5.256/kL	$30.78–$76.74	$52.70–$131.40	$105.40–$262.80
Melbourne, Yarra Valley Water, water and sewer usage (step 1 to step 3)	$3.831–$5.84/kL	$55.94–$85.27	$95.78–$146.01	$191.56–$292.01
Adelaide, SA Water, single dwelling (tier 1 to tier 3)	$2.52–$3.898/kL	$36.79–$56.91	$63.00–$97.45	$126.00–$194.90

Which tariff step will a partial drain land in?

Usually not the cheapest. SA Water’s tier 1 covers only the first 383.6 litres a day, and its average residential customer uses 189 kL a year, about 518 litres a day, so for an average household refill water lands in tier 2 at $3.597: $89.93 for 25 kL. Yarra Valley Water steps by litres per day across the bill: 25 kL in a 91-day quarter adds about 275 litres a day on its own. Water Corporation counts usage through a bill year that resets to tier 1, so a Perth refill early in your bill year is cheapest. Sydney’s single price makes it simple. The tiers are worked in full on what filling a pool costs.

Yarra Valley Water’s figures combine water and sewer usage; Sydney, Perth and Adelaide prices are water usage only. Fixed service charges do not change with a drain.

Why the tap water decides how much to drain

Dilution can only bring a pool down towards the water you refill it with. Sydney Water’s Potts Hill system averaged 128.3 mg/L dissolved solids over 2025–26 (typical 104–175 in April–June 2026). Water Corporation’s 2024–25 data puts South Perth and Kewdale at a mean of 477 mg/L and Two Rocks at 525. The same 2,500 → 1,500 correction therefore takes 42.2% of the pool in Sydney and 49.4% in South Perth.

Salt pools are deliberately far above 1,500 mg/L, so the dissolved-solids ceiling cannot apply to them as a number; NSW Health’s saturation index simply switches its TDS factor from 12.1 to 12.2 above 1,000 mg/L. The ceiling that matters in a salt pool is the chlorinator manual’s maximum salt, on what salt level a pool should run at. We found no Australian health document with a salt-pool TDS ceiling.

The refill-water rule: what comes out of the hose sets the floor, and the harder or saltier it is, the bigger the drain.

Should I fully drain my pool?

Rarely. Each worked trigger in the table is met by replacing 29–63% of the water, and emptying a shell risks lifting it where the ground is wet; what emptying and refilling a pool costs covers the water-table check and where the water may go, and draining to the sewer in NSW the discharge rule. Drain once by the full share rather than in several smaller rounds, which uses more water for the same result; how to lower pool stabiliser works that penalty. After rain, lower the level by the overflow or pump, as on how to lower pool water after rain.

Before you drain for combined chlorine, try a breakpoint shock (how often to shock a pool): NSW Health treats exchange as the fallback, and its effect is limited where tap water is chloraminated, as Sydney’s is (monochloramine typically 0.88 mg/L at Potts Hill taps).

When the water is not the problem

If calcium scale, etching or staining returns soon after a partial drain with the balance right (how to balance pool water), look at the surface and the equipment rather than draining again. Pool resurfacing cost by interior compares finishes, and you can compare pool quotes on the same written scope.

Common questions

How often should pool water be changed?

Only when a reading passes a ceiling that dilution alone fixes: stabiliser above 50 mg/L, calcium above its band, or dissolved solids above 1,500 mg/L. The NSW Health 2026 pool water guidelines manage these by exchanging water.

How much does it cost to partially drain and refill a pool?

For 25 kL at 2026–27 prices: $85.25 in Sydney, $52.70–$131.40 in Perth, $95.78–$146.01 in Melbourne (Yarra Valley Water, water and sewer usage) and $63.00–$97.45 in Adelaide.

What TDS level means I should drain my pool?

Above 1,500 mg/L in a non-salt pool; NSW Health says that can indicate lack of dilution. A salt pool is always above it; use the chlorinator manual’s maximum salt instead.

Do I need to drain my pool every few years?

No health department sets an interval. Test stabiliser, calcium and dissolved solids; if all are inside their ceilings, keep the water.

How much water should I replace to lower stabiliser?

1 − target ÷ reading. From 80 to 40 mg/L, half the pool: 25 kL of a 50,000 L pool.

Sources

General information only, current at 2026-10-20. Pool rules differ by state and by council, and penalty amounts are updated regularly. Confirm your own obligations with your council or a registered inspector before acting.