About 130 metres in length, Kathleen lies in wait about 30m underground at Barangaroo on the western edge of Sydney's central business district.

Within days, the giant tunnel boring machine will begin to churn through clay, sediment and rock on the first of two one-kilometre journeys under Sydney Harbour to build the second stage of the city's multibillion-dollar metro railway.

With four giant boring machines worming their way towards Sydney's CBD from Chatswood in the north and Marrickville in the inner west, the NSW government expects the 15.5km twin tunnels for the second stage of the rail line to be completed by March next year.

Transport Minister Andrew Constance, third from left, and Premier Gladys Berejiklian inspect the boring machine at Barangaroo. Credit:Bianca De March

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