Building below the bay: How a watertight basement was delivered beneath Sydney’s New Fish Market

By Ben Heffernan, Design Manager, New Sydney Fish Market

The new Sydney Fish Market at Blackwattle Bay stands as one of the most complex projects ever undertaken by Multiplex. A defining challenge was the construction of a basement car park located up to four metres below sea level - an environment demanding exceptional resilience against corrosion and hydrostatic pressure.

Designed to accommodate both parking and essential plant rooms, the basement required complete watertightness and structural integrity to withstand constant marine pressure. Even the smallest defect could lead to significant long-term issues.

To mitigate risk and enhance construction efficiency, Multiplex developed a full-scale prototype of the basement’s key construction and waterproofing details before commencing on-site works. This proactive measure focused on high-risk zones for water ingress and corrosion, allowing the team to test and refine construction detailing in a controlled setting.

The prototype underwent rigorous inspection by Multiplex’s Quality Assurance Committee, trade partners, and other stakeholders. It proved invaluable by surfacing design and coordination issues early and enabling quality improvements before full implementation. Lessons learned were documented in a comprehensive handbook shared across teams and subcontractors, reinforcing Multiplex’s commitment to excellence and knowledge sharing.

The result? A fully watertight, structurally sound basement that serves as the underwater car park for the new market. This approach not only improved construction outcomes and stakeholder confidence but also set a new benchmark for quality assurance in marine-based infrastructure.

Through early testing and close collaboration, we were able to reduce risk, improve reliability, and deliver a safer outcome in a highly challenging marine environment.