"It was only in September that we launched this first massive machine at Bella Vista, said New South Wales premier Mr Mike Baird on January 21. "In that time it has tunnelled 2.1km."

The $A 8.3bn ($US 6.6bn) project involves constructing 15km of twin-bore tunnels from Bella Vista to Epping and eight stations, with trains running at 4-minute headways during peak periods when the line opens in 2019.

"All four 900-tonne machines are now in the ground digging Australia's longest railway tunnels," added New South Wales transport minister Ms Gladys Berejiklian. "We are wasting no time delivering this world-class infrastructure project and doing it ahead of schedule and under budget. The project is now running $A 300m under budget and I am confident there will be further savings for taxpayers as the project progresses."