Final tests as launch nears for major under-city metro

Pairing extra homes with high-speed train lines has been touted as Sydney's solution to the housing crisis as a key part of Australia's largest public transport project nears its open date.

Full-service timetable testing has begun on the Sydney Metro City line, which provides the first underwater train crossing for the city's famous harbour.

The metro train line runs from Chatswood in Sydney's north, under the city centre and to Sydenham in the inner west and is expected to be open to the public in August, the NSW government said on Monday.

Hitting maximum speeds of around 100km/h, the train gets from North Sydney's Victoria Cross under the harbour to Barangaroo in just three minutes.

Premier Chris Minns joked that the experience of catching the underground train was closer to flying such was its impressive speed.

He added that public transport had to be at the forefront when planning future urban development in the nation's largest city.

The state government has ordered the snap rezoning of areas surrounding dozens of existing and planned train and metro stations in a bid to add hundreds of thousands of new homes.

"We're preparing for population growth, for a city where people live closer to the CBD and a more integrated city," Mr Minns said.

"For a long time, governments of different political persuasions have thought about public transport as an afterthought and housing as a primary source of economic drivers ... what we're trying to do is merge the two things."

The city metro project, planned and commenced under the former coalition government, has been hit with a series of cost overruns since it was approved in early 2017.

The final bill for the Sydney Metro City and Southwest line is expected to be about $21.5 billion, up $9 billion on its initial price-tag.

The last stage of the project involves an upgrade of the existing heavy-rail line between Sydenham and Bankstown, requiring a 12-month closure of the route.

It forms part of a wider rollout of metro services across the city, which combined constitute the nation's biggest public-transport project.

A Sydney Metro West line will connect the city centre with Parramatta, while a third project will connect St Marys in western Sydney to the region's new airport.

"We can't just have a situation where another street is added to the western fringe of Sydney every other week," Mr Minns said.

"We need to think more creatively and more intelligently about where people live and work, not just for the next few years but for the next two or three decades."

About 1000 people will be involved in "simulated scenarios" as would-be customers while the last tests are run on the line.

The 15.5km twin metro railway tunnels are up to 40m below the surface and link to several new stations in the city centre and inner south.