Labor's plan to scrap a project to convert Sydney's Bankstown rail line into one for single-deck metro trains if it wins the state election has become harder to achieve after the Berejiklian government vowed to get on with signing construction contracts.

The conversion of the existing 13-kilometre line between Bankstown and Sydenham gained planning approval on Wednesday, clearing the way for major construction work to start in the new year. The project is scheduled to take five years and includes upgrades to 10 stations.

A 13-kilometre stretch of the Bankstown line will be converted to carry driverless metro trains. Supplied

Transport Minister Andrew Constance said the government had begun signing contracts for the second stage of Sydney's metro project, which will cost up to $12.5 billion and includes the conversion of the Bankstown line.

"Work is under way. If the Labor Party want ... to cancel it, we will fight them on it," he said.

Mr Constance would not say how much it would cost taxpayers if construction contracts were cancelled, instead insisting that Labor release its costings on scrapping the project.

Transport Minister Andrew Constance and Premier Gladys Berejiklian at Sydenham station on Wednesday. AAP

"We are still signing contracts because this is what the community expects. They want governments to get on and deliver," he said.

But Labor leader Michael Daley accused the government of rushing to lock it into "their wrong priorities and bad decisions".

"It's bloody minded 95 days out from an election, 60 days from caretaker mode, for the government to be pressing the button on a billion-dollar decision," he said.

Mr Daley said he wanted to stop the conversion but "if the contract locks us in and costs an inordinate amount of public money to get out of it then we would be left no choice".

The future of the Bankstown line has been one of the clearest points of difference in transport policy between the Coalition and Labor ahead of the election in March.

Labor has argued that scrapping the line's conversion, as well as several toll road projects such as the proposed Beaches Link to the Northern Beaches, would free up money that would be better spent on a proposed metro rail line between Parramatta and central Sydney.

If the conversion of the Bankstown line does go ahead as the government plans, commuters will be forced to catch alternative transport during temporary closures for construction.

The disruptions will be most severe towards the end of the project in 2023 when the line will be closed for three to six months.

Mr Constance also indicated on Wednesday that the government is considering extending the metro line beyond Bankstown to Yagoona in order to find the best turn-back site for trains.

"We are looking at all the final options there in relation to the design," he said.