Alkimos: both TBMs are through, the marine works are done, and the job moves to fit-out

Tunnelling and marine works on the $2.8 billion Alkimos Seawater Desalination Plant were completed on 1 September 2026. Tunnel boring machines Mary and Karli finished a 15-month campaign, excavating up to 50 metres a day, 20 metres below the seabed, and installing around 32,000 WA-made concrete segments across more than six kilometres of tunnel.

The two drives are the longest desalination intake and outfall tunnels in Australia: a 2.5-kilometre intake running 1.6 kilometres offshore, and a 4-kilometre outfall running 3 kilometres offshore. Both were sized to allow expansion beyond the plant's first-stage capacity. The intake and outfall structures were installed by Beverley, Australia's largest registered jack-up barge.

The machines are now being demobilised, and marine exclusion zones have been lifted. The work now shifts to fit-out of the plant buildings and the start of electrical and mechanical works across the 29-hectare site — a different trade mix, and the point at which a civil-heavy job becomes a building-services one.

The Alkimos Seawater Desalination Plant remains on track to be fully operational by mid-2028.

In June 2026 the Alkimos Seawater Alliance announced a scope change, adding a groundwater treatment plant to the design of the broader desalination plant — a late addition to a project already two years into construction.

The essentials

Only Stage One is confirmed. The full scheme is two stages; the second is not committed.

Stage One scope:

• Desalination plant, 50 GL annual capacity
• Groundwater treatment plant (added June 2026)
• Ocean intake pipeline below the seabed
• Ocean discharge pipeline below the seabed, for brine
• 34-kilometre pipeline to transport drinking water to Wanneroo Reservoir

Water Corporation is expected to secure a power purchase agreement for up to 400 megawatts of renewable wind energy to power Alkimos along with its existing plants at Kwinana and Binningup.

Who is building it

Pipeline construction is 55 per cent complete and on track for completion by the end of 2027, ready to move water when the plant starts operating in mid-2028.

Splitting the pipeline between a pipe manufacturer for the first 6.5 kilometres and a civil consortium for the remaining 27.5 is an unusual carve-up, and worth understanding: the first section carries the most complex interface work at the plant end, while the balance is more conventional trunk main installation.

Who pays, and why it matters

The A$2.8 billion is funded by user charges. Not a government grant, not equity, not a Commonwealth contribution. Water customers.

That places Alkimos alongside a pattern now visible across Australian utility infrastructure. Transgrid sought more than $1 billion from electricity consumers for Project EnergyConnect, and the regulator refused it. CopperString is being restructured as a regulated asset recovering through network charges. Marinus Link's costs are allocated between Tasmanian and Victorian customers at 27.6 and 72.4 per cent.

Utility megaprojects are increasingly funded by the people who use them rather than by consolidated revenue — which means cost increases show up in household bills rather than in budget papers, and are scrutinised by economic regulators rather than parliaments.

For contractors the practical consequence is the same as on the transmission projects: a client recovering through regulated charges has less commercial latitude on variations than one spending appropriated funds, because every dollar has to be justified to a regulator.

The scope change is the thing to watch

Adding a groundwater treatment plant in June 2026, with construction underway since October 2024, is a significant design change.

Groundwater treatment is a different process train to seawater desalination — different pre-treatment, different membranes or media, different residuals handling. Integrating it into a plant already in detailed design and construction is not a matter of adding a building.

What is not public is how the change is being handled commercially. Whether it is a variation to the ACCIONA/Jacobs scope, a separate package, or a re-baselining of the project is the question a trade audience would want answered — and it is the sort of thing that only surfaces through the contractor or the alliance rather than through a media release.

What to watch

1. How the groundwater treatment plant addition is contracted, and whether it moves the 2028 date or the $2.8 billion.
2. Pipeline completion by end of 2027, which gates commissioning.
3. The 400 MW renewable power purchase agreement, which is a substantial energy procurement in its own right.
4. Whether Stage Two is committed, and on what trigger.
5. Commissioning through 2028 — as with rail, the tail of a water project is testing and process commissioning, not construction.

Published 23 September 2026