Western Harbour Tunnel will run beneath Sydney Harbour and is designed to relieve congestion and strengthen long-term transport resilience.
Reframing a critical transport challenge
Sydney’s road network is under sustained pressure. Existing routes through the CBD and across Sydney Harbour carry significant volumes of traffic each day, leaving limited redundancy when incidents occur and amplifying congestion across the wider network.
The NSW Government’s Western Harbour Tunnel (WHT) project will be a western bypass of the CBD, running underneath Sydney Harbour to connect the Warringah Freeway near North Sydney with WestConnex at Rozelle Interchange. Once completed, it will improve network resilience and support future growth across Greater Sydney. But delivering a six lane, 6.5-kilometre tunnel beneath one of the world’s most iconic and environmentally sensitive harbours poses major challenges.
The original reference design relied on an immersed tube tunnel, a tunnelling process which would have required extensive dredging, prolonged marine works, disturbance to the marine ecosystem and disruption to harbour operations. It would have introduced environmental risk, construction complexity and community impacts.
The challenge for Transport for NSW and delivery partner ACCIONA was clear: how to deliver a project of this scale while reducing risk to the harbour, minimising impacts on surrounding communities and maintaining confidence in constructability, cost and the program.
A tunnel solution designed to reduce risk and impact
AECOM, in a joint venture with Aurecon, supported ACCIONA to rethink how the harbour crossing could be delivered. Working as part of a fully integrated joint venture with Aurecon, we delivered a winning tender solution that helped shift the project from an immersed tube tunnel to a tunnel boring machine (TBM) solution for the harbour crossing.
Using two large diameter slurry TBMs, the design avoids any dredging of the harbour. This change significantly reduces disturbance to marine sediments and biodiversity, while also removing the need for temporary marine structures and cofferdams. Harbour operations, including ferries and commercial shipping, can continue unaffected during construction.
The preferred solution is one of the world’s largest TBM tunnels and the largest in Australia.
Designing Western Harbour Tunnel
After delivering the TBM tender solution, we were awarded the detailed design of most permanent work for Stage 2 of the Western Harbour Tunnel project.
The final design comprises:
- 6.5 kilometres of twin tunnels, each including a 1.5 kilometre TBM driven harbour crossing
- Underground TBM launch and receival caverns
- Fully underground slurry treatment plants
By refining road alignment and ventilation strategies, the team reduced tunnel lengths and cross sections. We also improved construction staging and constructability, reducing risk. Now in the construction phase, we’re providing subject matter expertise.
Once complete, the NSW Government’s Western Harbour Tunnel project is expected to ease the Sydney Harbour Bridge and Sydney Harbour Tunnel, improving network resilience and efficiency by providing a western bypass of the Sydney CBD.
Global collaboration, local outcomes
Delivering the Western Harbour Tunnel detailed design required deep tunnelling expertise across complex ground conditions. We mobilised a global team of more than 300 engineers and specialists across Australia, Asia and Europe, drawing on our international immersed tube tunnel and TBM experience while working closely with ACCIONA’s local delivery team.
Digital engineering was embedded throughout the project. Fully attributed 3D models were developed across disciplines to support design coordination, quantity tracking, construction staging and sustainability reporting.
Just as important was how the team worked. Parallel development of both the original immersed tube design and the alternative TBM solution during tender enabled informed decision-making under tight timeframes. Collaboration across time zones supported rapid iteration, challenge and refinement, turning early design risk into an opportunity for a more sustainable outcome.
Outcomes that matter beyond the tunnel
The Western Harbour Tunnel Stage 2 design demonstrates how major infrastructure can balance performance, environmental stewardship and community impact. By adopting a TBM-led solution, the project avoids dredging Sydney Harbour, reduces marine and shoreline disruption, and limits surface construction in densely populated areas.
From a transport perspective, the tunnel is designed to improve network resilience by providing an alternative harbour crossing and supporting future connections. For government and the community, this means a road network that’s better able to respond to growth, incidents and changing travel patterns over time.
For AECOM, the project reflects an approach grounded in collaboration, technical rigour and long term value, using global expertise to support better local decisions. The Western Harbour Tunnel is not just a new piece of infrastructure beneath the harbour; it is part of a broader system designed to keep Sydney moving while protecting what makes the city unique.