Under wraps: Sydney's first rail tunnel dug beneath harbour

It was a historic moment that passed Sydney by.

Without any public fanfare, a giant tunnel boring machine named Kathleen has completed digging the first rail tunnel under Sydney Harbour for a new metro train line.

The tunnel boring machine has been barged back to Barangaroo, where an underground station is being built.Credit: Wolter Peeters

So unremarked has been the occasion that Kathleen has already been pulled to the surface up a deep shaft on the harbour's edge at Blues Point, on the lower north shore, and barged back to where she began her journey at Barangaroo, on the western edge of the CBD.

In the coming weeks, the 130-metre boring machine will start on the second parallel tunnel.

The twin kilometre-long tunnels under the harbour are one of the most complex parts of the second stage of Sydney's metro rail line from Chatswood to central Sydney, and onto Bankstown. The first stage from Rouse Hill in Sydney's north-west to Chatswood opened in May.

Sydney transport planner Mathew Hounsell said the use of a specialised tunnel boring machine to bore under the harbour was a "very significant" engineering achievement.

Tunnel boring machine Kathleen being assembled at the site of the new Barangaroo metro station before she started tunnelling under the harbour.

"It is a significant milestone in Australian history. It demonstrates that we are capable of delivering major infrastructure projects like other countries - but it is not a world beater," he said.

He cited the 57-kilometre Gotthard tunnel under the Swiss Alps – the world's longest and deepest rail tunnel, which opened in 2016 – as an example of an extraordinary engineering feat.

Named after Kathleen Butler, who served as a technical adviser to renowned engineer John Bradfield on the construction of the Harbour Bridge, the boring machine is specially equipped to tunnel through water-saturated sandstone and sediment.

Kathleen at Barangaroo in June, shortly before she began digging the first rail tunnel under Sydney Harbour. Credit: Bianca De Marchi

In contrast, much of Sydney's road tunnel under the harbour was built almost three decades ago using pre-cast concrete tubes, which were lowered into a trench dredged on the seabed floor.

At Barangaroo, a deep hole has been excavated for the underground metro station there. It is one of four new stations in Sydney's CBD for the metro line, including at Martin Place, Pitt Street near Town Hall and Central Station.

The second stage of the line is due to open in 2024 and has been budgeted at up to $12.5 billion.

A Transport for NSW spokesman said Kathleen was in the process of being reassembled to start digging the second rail tunnel under the harbour. About 85 per cent of tunnelling for the new rail line between Chatswood and Sydenham had now been completed.

Kathleen was raised to the surface at Blues Point, on the lower north shore, after tunnelling under Sydney Harbour.Credit: Edwina Pickles