An extension is now underway to get people into the centre of Australia's biggest city, including a new tunnel crossing deep under Sydney Harbour.

As Australia's largest transport project gets underway, we go back under Sydney Harbour to check out history in the making as the construction of the twin metro rail tunnels continues deep under the surface.

After the completion of 30km of tunnelling, workers prepare to build railway stations, lay tracks and test driverless trains; at Central Station, Australia's busiest railway station, Central Walk, the underground pedestrian subway comes to life.

After almost 10 years of planning, building and testing, involving more than 60,000 people from around the world, Australia's largest transport infrastructure project nears completion.