Perth's desalination figures describe several different things: what a plant is capable of producing, how much it produced during a financial year, and how that water enters a larger supply system. Read the label attached to the number before comparing it with another figure. Plant descriptions and annual output accounts answer different questions.
What the capacity pages say
Water Corporation describes the Perth Seawater Desalination Plant at Kwinana as having capacity for 50 billion litres a year. Its page says production began in 2006. The Kwinana plant overview gives readers the scale of the facility; it does not attach that capacity figure to one year's actual output.
The utility's Southern plant overview describes capacity of 100 billion litres a year at Binningup. It also explains that pipelines and a pumping station were added to deliver water into the Integrated Water Supply Scheme. Keep those two plants separate when reading their names in a report. "Perth" in a plant name does not mean every desalination facility serving the region.
Reading the output table
The 2026 annual report covers the financial year ended 30 June 2026. Its metropolitan supply table records 46,354 megalitres from the Perth plant and 103,285 megalitres from the Southern plant. Combined desalination output is 149,639 megalitres. The table is on printed page 72 of the 2026 annual report; the introduction specifies its reporting period.
One megalitre is one million litres. On that basis, the recorded Perth output is 46.354 billion litres, and the Southern output is 103.285 billion litres. The reporting period and measured volume stay the same after conversion. If you save a figure to compare with a later report, save its plant name and financial-year end beside it.
The Southern figure is above the published 100-billion-litre capacity description on the plant page. These sources do not explain the difference. It would be unjustified to treat that comparison as proof of an operating fault, a new expansion or a precise capacity-utilisation rate. The public description and annual output line are different records. A closer comparison needs Water Corporation to explain the engineering basis.
A regional supply figure has a boundary
Perth's water-supply overview describes desalination alongside groundwater and groundwater replenishment as the region adapts to less dependable rainfall into dams. A regional source total cannot identify the blend arriving at an individual address. Keep the regional explanation separate from questions about the particular supply serving a hotel or home.
The annual table also has separate lines for bulk exports and operational use. Its desalination total is therefore not automatically the same measure as the total potable water supplied to customers. Before calculating a share, check that the numerator and denominator describe the same system, period and accounting boundary.
Future capacity needs a future date
Water Corporation's supply plan describes Alkimos's first stage as capable of supplying 50 billion litres a year, with readiness planned for 2028. It describes full capacity as 100 billion litres a year. Those are future planning statements, not additional measured production to add to the 2025-26 table.
For a visit, the Perth city guide is a starting point for general drinking-water guidance. Desalination volume figures explain supply infrastructure. They provide no sample result for your accommodation's tap, and cannot replace local instructions about a specific water problem.
