The $200 million promise to start major construction works on the Parramatta Light Rail extension to Sydney Olympic Park by 2027 comes after the project's completion time blew out to 2031.
Labor is suspicious the government's only monetary pledge currently - two bridges including one over Parramatta River - means the project is in danger of being scrapped in favour of busways.
"We're not convinced the government will get on with it, given they've promised it since 2014," Opposition leader Chris Minns said on Friday.
Mr Minns said the 10-kilometre line, likely to cost more than $2 billion, was a "must" for high-density suburbs on the route, including Wentworth Point which currently relies on buses and private vehicles.
If elected in March, Labor would allocate money in the forward estimates to the project and begin major construction before the 2027 election.
But Mr Minns wouldn't commit to a completion date, only saying it would be completed "not long" after 2027 and earlier than 2031.
"We can't promise miracles, but we are consistent about the money that will be allocated and the project will be completed," he said.
The trams would also contain 30 per cent Australian-made parts to fit Labor's local content rule for major projects.
"We think that that's modest and, having spoken to experts in this field, Australian firms can compete with those rules in place," Mr Minns said.
Business Western Sydney executive director David Borger welcomed the announcement to connect "stranded communities" who lacked good public transport and certainty.
Parramatta's population is expected to increase by 127,000 over the next 15 years, while nearby areas of Blacktown and the Hills will need to home another 110,000 people each.
"Places like Wentworth Point are a bit like someone sliced up a bit of Manhattan and floated down the Hudson River and it arrived in the middle of Sydney," Mr Borger said.
"It doesn't work without great public transport."
The project is currently languishing in the planning stages, with Transport for NSW due to respond to public submissions by March 30.
Among the objections, Parramatta City Council and the Environmental Protection Agency have pushed against a proposal to extend construction hours to 7am to 7pm every day including Sunday.
Labor's commitment comes after it announced the candidacy of Lord Mayor Donna Davis for Parramatta on Wednesday, signalling its intention to win back the large multicultural seat from the Liberals.
Liberal MP Geoff Lee, who holds Parramatta by a 10 per cent margin, will retire at the next poll and the Liberal party is yet to name its candidate.
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