Commuters north-west of Sydney will soon have a direct train line to the city when the latest instalment of the Sydney Metro is up and running in August.

A trial run of the full-service timetable has begun between Chatswood, under Sydney's Harbour and out to Sydenham, and will cut travel time for thousands of commuters when it opens.

Premier Chris Minns said it will open to the public on an undisclosed date in August, after a year of intensive testing and the building of new tunnels, trains and stations. 

"The countdown is now on to thousands of people across Sydney getting to spend less time travelling and more time with their family and friends," Mr Minns said.

The premier said Sydney's Metro transformation is a major key to building more houses near transport infrastructure. 

Transport Minister Jo Haylen said the new line would have a significant impact for those living north-west of Sydney.

"We anticipate the largest increase in passengers for this new part of the metro will come from the-north west," she said. 

"There's many people living there right now that might not choose the metro because of the change they need to make at Chatswood.

"This service will be on the same train all the way from Tallawong, through the CBD to Sydenham."

After the full service begins from August, travel times between Martin Place and Waterloo should take six minutes, Sydenham to Macquarie University will take 33 minutes, Central to Chatswood will take 15 minutes, and North Sydney's Victoria Cross to Barangaroo will take three minutes. 

The next stage of the metro will create an extension of services from Sydenham to Bankstown, meaning the T3 Bankstown line will be closed for a year, before it opens in 2025.