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.