Western Harbour Tunnel Reaches Major Milestone with Caverns Completed

At a Glance

• Twin underground caverns have been excavated in Sydney’s inner west to launch huge tunnel boring machines for the Western Harbour Tunnel.

• Each cavern stands 28 metres high – big enough to hold 22 Olympic pools – and will house a 137 m long, 4,300 tonne boring machine to dig a 1.5 km road tunnel under Sydney Harbour.

• The multibillion-dollar project is slated to open in 2028, aiming to cut traffic on the Harbour Bridge by about 17% and trim some cross-city commutes by up to 20 minutes.

• Built as a publicly owned infrastructure asset, the tunnel is supporting around 7,000 construction jobs and promises to reshape Sydney’s road network with faster, more reliable travel.

Construction crews have finished carving out two massive underground caverns at Birchgrove, marking a critical breakthrough for Australia’s newest road tunnel. These twin caverns – each vast enough to fit six full-size Sydney ferries – will serve as launch chambers for the tunnel boring machines (TBMs) that will dig the Western Harbour Tunnel beneath Sydney Harbour. The caverns are located just 230 metres from the harbour’s edge and tower roughly 28 metres high, making them among the largest excavations of their kind ever completed in Australia. 

New South Wales officials hailed the achievement as a key step toward delivering Sydney’s first new road harbour crossing in over three decades. “Today, the Minns Labor Government is one step closer to delivering the Western Harbour Tunnel,” said NSW Roads Minister Jenny Aitchison. “This is a once-in-a-generation project, with the Minns Government on track to deliver Sydney’s third road harbour crossing.” Local leaders emphasized that unlike many recent toll road projects, this tunnel will remain in public ownership. “After 12 years of […] privatisation, we are building this project as a public asset for the people of NSW,” Aitchison noted, underscoring the government’s commitment to retain control of the vital infrastructure.

Colossal Tunnel Boring Machines Set to Launch 

With the caverns now complete, attention turns to assembling and launching the two colossal TBMs that will excavate a 1.5 km route under the harbour from Birchgrove to Waverton. Each machine measures about 137 metres in length and weighs over 4,300 tonnes – roughly the weight of 88 double-decker buses. These TBMs are the largest ever deployed in the Southern Hemisphere and will actually be assembled underground within the new caverns, an engineering feat in itself. Crews are using a 500-tonne gantry crane to piece together the TBM segments, which were shipped from overseas in 263 major parts (plus another 125 container loads of components). Once fully built and launched, the enormous borers will mine through Sydney’s sandstone around the clock, guided by about 40 workers per shift. 

Officials describe the scale of the machinery and the precision required as “breathtaking.” “These giant caverns are vast enough to fit six Emerald-class Sydney ferries each, but once the TBMs are assembled inside them, there will only be around one to two metres between each TBM and the chamber walls,” Minister Aitchison explained, highlighting the tight tolerances. Over 4.8 million tonnes of rock and soil have already been excavated across the project to date, and the TBMs’ year-long under-harbour drive is expected to unearth even more before breaking through on the north shore. According to project engineers, tunnelling under the harbour is one of the most complex stages of the build – often likened to assembling a gigantic Meccano set deep underground. 

Barring any unexpected delays, the tunnel boring machines are scheduled to begin digging under the harbour by the first half of 2026. The Western Harbour Tunnel’s overall tunnel excavation is now well past the halfway mark, combining this new under-harbour section with earlier tunnel works already completed from Rozelle towards the harbour. “The Western Harbour Tunnel TBMs are the largest in the world to be assembled underground,” Aitchison said, “and our team is now preparing them for launch in the coming days.” Once underway, the TBMs will grind forward day and night, likely taking about 12 months to complete the 1.5 km crossing beneath Port Jackson’s seabed. 

Reduced Congestion and Faster Journeys Ahead 

When the Western Harbour Tunnel opens to traffic (targeted for 2028), it will form a seamless 6.5 km link between the existing Warringah Freeway at North Sydney and the new Rozelle Interchange in the inner west. This additional harbour crossing is designed to relieve chronic traffic bottlenecks on Sydney’s current routes. Planners estimate the new tunnel will remove thousands of cars from the overburdened Harbour Bridge, Harbour Tunnel, and city feeder roads each day. In fact, traffic modeling forecasts a 35% reduction in volumes on the Western Distributor, a 20% drop in the Harbour Tunnel, and about 17% less traffic on the Sydney Harbour Bridge once the new route is in operation. By providing an alternative route bypassing the city centre, the project is expected to shave up to 20 minutes off peak-hour trips between points like North Sydney and western suburbs such as Leichhardt or Sydney Olympic Park. 

Beyond the convenience for drivers, the tunnel’s impact extends to Sydney’s economy and environment. Smoother traffic flow and shorter trip times mean productivity gains and reduced vehicle emissions from idling in congestion. Crucially, the Western Harbour Tunnel is being delivered as a public infrastructure asset rather than a privately tolled concession. This approach aims to ensure that any long-term benefits – from travel time savings to operational revenue – “stay with the people of NSW,” as officials put it. Keeping the tunnel in public hands may also give the government flexibility in managing tolls or usage to maximize network benefits for commuters. 

Project Timeline, Investment and Jobs 

The Western Harbour Tunnel was first announced in 2017 as part of an integrated transport plan to expand Sydney’s road capacity. After years of planning and early works, major construction began in 2022. The project is moving forward in two main stages: the southern tunnels to Birchgrove (Stage 1, now largely finished) and the harbour tunnels north to Waverton (Stage 2, currently in progress). Together, these works represent one of Australia’s largest infrastructure investments. Including development and enabling works, the tunnel’s total cost is estimated at nearly $7.4 billion, reflecting complex engineering and some scope changes since inception. The government’s latest budget confirmed funding to see the project through, and construction remains on schedule despite the significant challenges of tunnelling in a busy urban environment. 

Building the Western Harbour Tunnel is also a major jobs generator. Around 7,000 jobs are expected to be supported over the life of construction, from tunnelling crews and engineers to suppliers and support services. NSW officials have praised the skilled workforce driving the project. “NSW has the premier underground construction workforce anywhere in the world and we want to thank the thousands of workers for what they have achieved so far,” said a previous Roads Minister during a progress update. The expertise honed on projects like WestConnex and Sydney Metro is being brought to bear on this tunnel, helping to overcome technical hurdles and maintain safety beneath the harbour. 

Final Thoughts 

With the gigantic TBMs soon set to begin their subterranean journey, Sydney’s Western Harbour Tunnel is on track to open by 2028. Upon completion, this publicly owned motorway tunnel will stand as a transformative addition to Sydney’s infrastructure – one poised to untangle traffic snarls, connect communities across the harbour, and bolster the city’s growth for decades to come.