- !Perth’s annual dam inflows have fallen from ~420 billion litres in the 1970s to ~77 billion litres today — an 80% collapse driven by climate change reducing winter rainfall across the southwest.
- →Perth’s water now comes from four sources: desalination (~37–47%), groundwater (~46%), surface water (~16% of urban supply), and groundwater replenishment (~3–8%). A third desalination plant at Alkimos is under construction.
- →Groundwater is hard; desalinated water is soft. The blend of these sources in your suburb determines your actual tap water hardness — which varies from below 100 mg/L to above 300 mg/L between Perth suburbs.
- ✓Groundwater replenishment (recycled wastewater) undergoes ultrafiltration + RO + UV before entering aquifers — the highest-treated source in the network.
- →All Perth scheme water uses free chlorine. Long networks require maintained residuals — a carbon block filter is the practical intervention for taste and odour.
A supply that has fundamentally changed
Most Perth residents know their water comes from "the dams" or "the tap." What fewer understand is that Perth’s water supply has undergone one of the most dramatic transformations of any city in the developed world over the past 50 years. The water coming out of a Perth tap in 2026 has a fundamentally different origin than it did in 1975, and that difference has direct consequences for hardness, mineral content, and what filtration needs to address.
The streamflow collapse
Until the mid-1970s, Perth relied overwhelmingly on rainfall flowing into the Darling Scarp catchment dams. Annual streamflow averaged approximately 420 billion litres per year. Then Perth’s climate changed — average winter rainfall declined, and crucially, the character of rainfall shifted toward more intense summer events and less of the steady soaking winter rain that saturated soil and produced catchment runoff. Dry soils absorb more rain before runoff begins.
| Period | Average annual streamflow into IWSS dams | Change from 1970s baseline |
|---|---|---|
| 1960s–mid 1970s | ~420 billion litres/year | Baseline |
| Late 1970s–1990s | ~180–250 billion litres/year | −40–57% |
| 2000s–2010s | ~80–120 billion litres/year | −71–81% |
| 2020–2024 average | ~77 billion litres/year | −82% |
| 2023–24 | Lowest on record | Ongoing decline |
An 80% reduction in streamflow is an existential challenge for a city that once relied on it for most of its water. Perth’s response has been to develop alternative sources at a scale and speed that makes it one of the world’s most studied examples of urban water adaptation.
What Perth’s water is made of today
As of 2023–24, Perth’s Integrated Water Supply Scheme (IWSS) drew from four distinct sources:
| Source | Approx. share of IWSS | Quality implications |
|---|---|---|
| Desalinated seawater (Kwinana & Binningup) | ~37–47% | Very low TDS, very low hardness. Near-pure before remineralisation. Chlorinated post-treatment. |
| Groundwater (Gnangara & other systems) | ~46% | Mineral-rich. Hardness 80–300+ mg/L by location and aquifer. Iron, manganese possible. |
| Streamflow into dams (surface water) | ~16% of urban supply | Lower minerals than groundwater. Now a minor contributor. |
| Groundwater replenishment (recycled water) | ~3–8% | Ultra-pure post-treatment (ultrafiltration + RO + UV). Essentially mineral-free before entering aquifer. |
A third desalination plant at Alkimos is under construction, expected operational by 2028 with an additional 50 billion litres per year capacity, pushing the desalination share higher still.
What this means for water quality at your tap
Desalinated water — very soft: The industrial-scale RO process removes virtually all dissolved minerals from seawater. Even after remineralisation for corrosion control, desalinated water entering the network is very low hardness — typically below 50 mg/L CaCO₂.
Groundwater — hard and variable: Perth’s groundwater sits beneath limestone and mineral-rich geology. Water moving slowly through this rock dissolves significant calcium and magnesium. The hardness varies dramatically by aquifer and bore location — from below 100 mg/L to above 300 mg/L.
Blending — the source of suburb-to-suburb variation: Water Corporation blends desalinated water with groundwater in the distribution network, with the blend varying by season, demand and network zone. This blending explains why Perth water hardness varies so significantly between suburbs — suburbs closer to desalination plant feed points may receive softer water; groundwater-dominant zones are harder. The blend changes seasonally, making a single number for "Perth hardness" misleading. Our Perth hardness by suburb guide shows the actual range.
Water Corporation publishes real-time dam levels, streamflow data and annual drinking water quality reports at watercorporation.com.au. Annual reports break water quality by locality. The Bureau of Meteorology's National Water Account provides independent supply figures by source.
What this means for filtration in Perth
Hardness from groundwater dominance: The high groundwater share drives Perth’s hardness problem. This is not a water quality failure — it is a consequence of the source geology. Melbourne’s soft water comes from protected mountain catchments. Perth’s groundwater comes through limestone. The practical implication is that TAC or softening is more justified in Perth than in any other Australian capital.
Desalination and long-term trends: The increasing desalination share will gradually soften Perth’s supply over time as the Alkimos plant comes online. Some suburbs may notice a very gradual softening trend over the next decade. This does not eliminate the hardness issue in groundwater-dominant zones.
Chlorine: All Perth scheme water is chlorinated with free chlorine (not chloramine). Long distribution networks require maintained residuals. A carbon block filter remains the practical intervention for chlorine taste and odour.
Groundwater replenishment: The 3–8% of supply from the GRS undergoes ultrafiltration, RO and UV before entering the aquifer — far more treatment than any other source. This component does not introduce quality concerns.
Perth now draws the majority of its water from groundwater and desalination — sources with fundamentally different mineral profiles. The high groundwater share drives Perth's hardness problem. The desalination share introduces very soft, low-mineral water. The blending of these two creates the suburb-to-suburb hardness variation unique to Perth.
Before choosing a filter in Perth, check your suburb's actual hardness. TAC or whole-home softening is strongly justified in high-hardness zones. Use our Perth hardness guide and comparison tool to match the filter to your supply.