Subiaco's AUD 105 Million Design Phase Carries a Regional Investment Test

Design funding advances the compact-plant option

Water Corporation's 22 October announcement allocates an initial AUD 105 million for detailed design, engineering and site preparation. It reports current treatment of about 65 ML/day and proposes around 23 ML/day additional capacity [1]. Infrastructure WA's business-case assessment, dated December 2025 and published in June 2026, specifies a 90 ML/day new compact plant against a 67 ML/day base case. Its roughly AUD 1.3 billion whole-project estimate includes demolition, escalation and contingency. The estimate is based on Class 4 design with a 32% P90 contingency [2].

Deferred network works carry much of the benefit

IWA gives the preferred option a benefit-cost ratio of 1.2. Nearly half its monetised benefits come from deferring more than AUD 1.1 billion of Woodman Point treatment and Sepia Depression Ocean Outlet Landline upgrades. IWA therefore calls for the feasibility and timing of that deferral to be defined during subsequent development [2]. Subiaco's commissioning sequence affects investment elsewhere in the system. A later available treatment train could shorten the deferral period and reduce that benefit, even if the new plant ultimately delivers its intended capacity.