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.

With 30 kilometres of tunnelling finished, it's time to start building railway stations, laying tracks and testing driverless trains. At Australia's busiest railway station, Central Station, the pedestrian subway known as Central Walks comes to life.

With the construction of Australia's biggest public transport project in full swing, we head back under Sydney Harbour and witness the historic completion of the twin metro rail tunnels deep below the surface.

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.