Sources & methodology

Storage levels

The combined percentage is total stored volume divided by total listed capacity. Individual reservoir percentages are calculated in the same way, rather than averaging their percentages. Thomson accounts for about 59% of the system’s capacity.

Daily volumes are reported in megalitres (ML). We show large volumes in gigalitres (GL): 1 GL = 1,000 ML = 1 billion litres. Changes between percentages are shown in percentage points.

The monthly comparison uses the same date in the previous calendar month, or that month’s last day when it is shorter. It subtracts the published daily percentage for that earlier date from the current percentage rounded to two decimal places. An unavailable comparison is shown as a missing reading, never zero.

Water transfer view

The transfer view shows a sourced subset of documented connections among the major reservoirs and incoming sources. Arrows show available directions, not live flows. The line shape is schematic; it can combine conduits, aqueducts, pumps and treatment stages, and is not a map of physical pipe alignments. Select a reservoir for its connection list and source links. The treatment and customer-supply rows below the map show the main outlets and local branches consistently; supply areas overlap and water can be blended or redirected.

Cardinia can pump water back to Silvan. The North–South Pipeline is a conditional supply route, shown dashed. Natural catchment inflows are omitted. Tarago fills from its own catchment and supplies customers through Tarago Water Treatment Plant, with a separate untreated branch to Gippsland Water’s own plants. Thomson and O’Shannassy reach treatment through the Upper Yarra–Silvan and Silvan systems respectively. Greenvale receives treated water and applies further disinfection; the Silvan–Cardinia transfer bypasses treatment at Silvan. Sources include Melbourne Water’s transfer overview, reservoir pages and its 2025–26 water-quality report. Download evidence for each displayed connection.

Individual reservoir history

Each reservoir includes 60 month-end observations from November 2021 to October 2026, obtained from Melbourne Water’s daily public feed and checked against the requested dates. The current observation is appended separately. The chart switches between percentage full and stored volume in GL. Capacity means the listed maximum volume; the line shows the water actually stored. Missing periods are not joined across gaps longer than 45 days. Historical points are a curated snapshot, with a separate current reading.

Historical comparisons

The source provides monthly snapshots from June 1948. Values are aligned using each observation’s actual calendar month. A missing month is left blank; the line breaks rather than turning missing data into zero. The latest incomplete month keeps its actual observation date, and the current year ends at the latest published point.

Historical percentages refer to capacity at the time. Reservoir additions and changes therefore create breaks in comparability. In particular, Thomson joined the system in July 1984. Today’s listed system capacity of 1,812,175 ML has applied since July 2010. Melbourne Water’s explanation and capacity-change table sets out those changes.

The four most recent years are selected by default; compare at most four years to keep the traces legible. A separate, optional 2009 drought reference provides context for the lowest monthly reading in the record. Line styles and labels supplement colour. A month slider, data table and CSV download provide exact values without relying on hover. In the full-history view, drag the range handles or choose start and end calendar years to zoom. The overview retains the complete record, while the main chart, reading selector, table and CSV follow the selected range. The percentage axis continues to start at zero.

Recent weekly water use

The recent-use panel reproduces Melbourne Water’s published weekly litres-per-person figure and the reported week-ending date. Four consecutive weeks are checked alongside storage readings in the same scheduled daily task. Page visits read the saved snapshot and do not contact the source. The four-week average is the arithmetic mean of four equally long weekly periods, using the source’s already-rounded values; it is shown as approximate and is not a calendar-month estimate.

This is a per-person indicator, not per customer account or an individual meter reading. The source does not publish a residential/business split with the weekly observations. Its data explainer and annual reports describe different water-use measures without a complete weekly calculation formula. We therefore retain a neutral weekly label and do not apply a Target 150 benchmark to this panel. Compare these weeks with one another; the annual customer charts below are a separate aggregation with their own definitions.

Water use: matching the numerator and denominator

The total is billed residential plus non-residential customer water use, excluding recycled water. It covers Greater Western Water, South East Water and Yarra Valley Water. Before July 2021, City West Water and Western Region Water are included as Greater Western Water’s predecessors. This is a custom aggregation of reported utility data, not an official BOM-published Melbourne series.

Network losses, other non-revenue water and supplies to other retailers are excluded. This is why the series differs from Melbourne Water’s larger bulk-supply figures.

The stacked bars show residential and non-residential volumes separately. “Business & other” includes non-household categories such as public services. There is no industry-level breakdown, including data centres, in these indicators. The Residential only option on the per-resident chart divides residential volume by the same reported service population and year length.

For each financial year, the per-person measure uses exactly the same customer-use volume, divided by the same retailers’ reported service population and the actual number of days in that year. Leap years use 366 days. Business use is allocated across that population, so the result is not an estimate of an individual’s household consumption and should not be compared directly with Target 150.

L/person/day = customer ML × 1,000,000 ÷ service population ÷ days

Indicators: W8.3 (residential consumption), W9.3 (non-residential consumption), and C1 (population). Earlier C1 values are in thousands and are converted to people. Source workbooks: 2024–25 complete dataset and 2020–21 dataset for the predecessor retailers. BOM’s indicator definitions explain the measures.

Earlier records: 2005–06 to 2016–17

The annual-use page also has a separate Earlier records view with 12 complete financial years from BOM’s 2016–17 dataset. It sums W8 (residential supply) and W9 (commercial, municipal and industrial supply) for City West Water, Western Water, South East Water and Yarra Valley Water. Those definitions include recycled water and stormwater, and their business coverage differs from the newer W9.3 category. The two periods are therefore displayed separately, with no connecting line or whole-period percentage change.

The same four retailers supply the reported C1 population denominator, converted from thousands to people. Their service areas are not a fixed Greater Melbourne boundary; Western Water includes areas outside metropolitan Melbourne. Estimates remain as reported, without harmonising earlier censuses or boundaries. Western Water’s reported population falls from 163,401 to 153,358 in 2016–17; the workbook does not explain the decrease. It should not be read as proof of actual demographic decline.

All 144 required utility/indicator/year observations are present. Earlier split indicators contain inconsistencies, so they have not been used to manufacture a supposedly comparable non-recycled series. 2005–06 is the earliest complete year verified in the checked workbooks, not a claim that no older records exist. Download the earlier data, exact workbook cells, definitions and population flags.

Desalination: volume and percentage

The percentage view divides each year’s desalinated volume by Melbourne Water’s total annual bulk water supply (BOM W11.3) for the same financial year. This is a measure of relative scale, not the fraction consumed at household taps. The denominator is broader than the metropolitan customer-use series. For 2025–26 it uses the report’s approximately 504 GL total supply, giving approximately 11.6%.

For 2016–17 to 2024–25, the chart uses Melbourne Water’s reported W3.1 net desalinated water sourced, based on treated-water production/outflow. For 2025–26, the striped bar uses 58.3 GL reported as supplied by the Victorian Desalination Plant in Melbourne Water’s Water Quality Annual Report, page 5. The last observation changes source and is rounded to 0.1 GL.

An order is a commitment for supply, not proof of the volume produced in a financial year. Production can cross an order-year boundary. Water can also remain in storage, so supply in a given year does not tell us how much desalinated water was consumed at taps that year. Cumulative mode adds the reported annual volumes; it does not estimate the amount still in storage.

Research behind the design

This is a working design hypothesis informed by desk research and a review of the existing desktop and mobile site. It has not yet been tested with residents, and it is not a certified accessibility audit.

• WaterNSW WaterInsights: complementary map and list views.
• Water Corporation dam levels: seasonal context, tables and downloads.
• Seqwater dam levels: visible observation times and operating context.
• ONS line-chart guidance: meaningful comparisons and honest handling of missing observations.
• W3C accessibility guidance: labels and line styles so colour is not the only cue.

Sources and reuse

Download the annual data and source provenance (JSON). This includes workbook cells, unrounded values, source URLs, calculation definitions and known quality flags.

DataVic also lists Melbourne Water’s historical daily reservoir dataset under CC BY 4.0. Its ArcGIS complete-record service is an older archive, not the current daily feed used here. A public commercial service will need a confirmed data-access arrangement and a review of the applicable reuse terms for each source.

This prototype is independent and is not endorsed by Melbourne Water or the Bureau of Meteorology.