About 40 metres below the surface at Concord in Sydney's inner west, a giant cavern more than five metres high and almost 12 metres wide gives a sense of the scale of the $16.8 billion WestConnex project.

Some 21 boring machines have been working around the clock for more than a year to carve out about 1 million cubic metres of sandstone, much of which has ended up at housing development sites in the city's west.

Australia's largest infrastructure project marked a milestone on Wednesday when a road header bored through rock to connect two sections of tunnels which, by 2019, will have thousands of vehicles passing through it every day.

About 70 per cent of the excavation for the 5.5-kilometre motorway tunnels has now been completed.