ACCIONA, as part of the Alkimos Seawater Alliance, has completed the construction of two undersea tunnels for the Alkimos Seawater Desalination Plant in Perth's north, marking a major milestone in the $2.8 billion project that will help secure Western Australia's drinking water supply amid declining rainfall.

Extending kilometres offshore

The tunnels, which will house marine intake and outfall pipes, extend 1.5 kilometres and 3 kilometres offshore. The longer tunnel will discharge brine far enough into the ocean to allow safe dispersion without harming marine life. This stage of the project was finalised last week following 15 months of continuous tunnelling operations.

Mary and Karli: the machines behind the milestone

Two tunnel boring machines, named Mary and Karli, were used to complete the works. Crews operated the machines around the clock in rotating 12-hour shifts, sometimes working 20 metres below the seabed. The next phase will involve connecting the tunnels to the intake and outfall structures that will channel seawater to and from the plant.

A climate-independent water source for Perth

Once operational in mid-2028, the Alkimos Seawater Desalination Plant is expected to produce approximately 50 gigalitres of drinking water annually, equivalent to 50 billion litres. This output will play a critical role in supplementing Perth's water supply as traditional sources continue to decline.

The plant will provide a climate-independent water source for Perth's growing population and reduce reliance on rainfall-dependent dams. Construction will now focus on above-ground facilities and integration works, with the project scheduled to be fully operational by mid-2028.

Minister highlights the significance

Hon. Daniel Pastorelli, West Australian Minister for Water, highlighted the significance of the milestone in the context of Western Australia's changing climate.

"This winter, if you've experienced it here in Perth, has been particularly dire," Mr Pastorelli said.

"Stream flow into our dams is now at a 10-year low, with only a little over 10 billion litres reaching our dams on the Darling Scarp this year, compared with an average of about 75 billion litres over the last decade."

"As a result, this winter just proves how important that investment decision was," Mr Pastorelli said.

The completion of the tunnels represents a significant step toward delivering one of Western Australia's largest water infrastructure projects.