Australia's largest rail project, the Sydney Metro, will get a significant chunk of infrastructure funding in Tuesday's NSW budget.

The Baird government will commit $12 billion over the next four years towards the rail line which runs from Chatswood to Bankstown, via the CBD.

Both stages of the Sydney Metro City & Southwest and Sydney Metro Northwest are now fully funded, Premier Mike Baird said on Monday.

The government has not yet released the final cost of the whole project but it's expected the price tag will be close to $20 billion.

The 2016-17 allocation of budget funding will go towards building eight new metro stations in the city's northwest and maintaining construction of the four-kilometre skytrain at Kellyville.

Money will also be allocated to the tender process for the twin 15.5km tunnels to be built under Sydney Harbour, with demolitions expected to take place as early as next year.

Sydney Metro services in the city's northwest are expected to start running in the first half of 2019, while those in the city and southwest suburbs will run from 2024.