Kathleen's key job in historic crossing of Sydney Harbour

Within months, a 130-metre boring machine named Kathleen will begin one of the most complex parts of tunnelling for Sydney's multibillion-dollar metro railway.

And in the process, she will make history.

A part of the tunnel boring machine named Kathleen is lifted onto a barge from a ship at White Bay.Credit: NSW government

Churning through clay, sediment and rock, the 975-tonne borer will dig the first rail tunnels under Sydney Harbour from Barangaroo, on the western edge of the CBD, to Blues Point on the lower north shore. The twin kilometre-long tunnels will form part of the second stage of a $20 billion metro rail line that will transport passengers in driverless, single-deck trains.

After arriving at Sydney's White Bay from China last week, the fifth boring machine for the second stage of the rail line was barged to the site of a new metro station at Barangaroo for assembly.

The borer is specially designed for the different ground conditions under the harbour, and will use pipes and fluids to control pressure in the machine by turning spoil into slurry and pumping it out.

It will start the journey under the harbour in the coming months from a large cavern next to the site of Barangaroo station, one of four to be built in the CBD for the metro line.

Kathleen has been designed especially for the ground and rock conditions found at the bottom of Sydney Harbour.

After digging the first tunnel, Kathleen's giant cutter head and main section will be lifted at Blues Point onto a barge and returned to Barangaroo. Her support trailers will be pulled back through the tunnel to Barangaroo.

Kathleen will then begin digging the second rail tunnel under the harbour before she is again lifted out at Blues Point, near Harry Seidler's well-known Blues Point Tower, and shipped to her next job. Before she arrived in Sydney, German company Herrenknecht assembled and tested the boring machine at its factory in China.

Transport Minister Andrew Constance said the fifth boring machine for the second stage of the metro line was named after Kathleen Butler, who had a key role in the construction of the Harbour Bridge as the technical adviser to the city's renowned engineer John Bradfield.

Parts of Kathleen are barged to Barangaroo where she will be assembled before she starts digging two kilometre-long tunnels under Sydney Harbour.Credit: NSW government

Underground workers have a tradition of giving boring machines female names as they look to Saint Barbara for protection.

Four boring machines are already well into the job of churning through thousands of cubic metres of earth to form 15.5-kilomtre twin tunnels for the second stage of the metro line, which will run from Chatswood to Sydenham in the south, and onto Bankstown in the west.

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

The government has said it expects the tunnels for the second stage to be finished within 18 to 24 months. The first stage from Rouse Hill in Sydney's north-west to Chatswood is due to open to passengers in May at a cost of about $8 billion.