The Herald revealed at the weekend that "cabinet-in-confidence" documents show the cost of the Baird government's signature transport promise for western Sydney is estimated to be more than $3.5 billion.

The government has committed $1 billion in funding for the 22-kilometre line stretching from Westmead to Strathfield via Parramatta and incorporating Carlingford, Camellia and Olympic Park. But it has yet to decide on the rest of what is expected to be a mix of so-called value capture and additional contributions from the public purse to pay for the new line.

Defending the benefits of the proposed light rail line, Mr Constance said he would not pre-empt the means of funding the project until the final cost was determined next year.

"We have not released any costings in relation to this because we have not bedded down the route; we have not bedded down the finalised design; and as result we have not finalised the cost," he said.

He confirmed that the costings revealed by the Herald were from internal Transport for NSW documents but said "there is no cost blowout because there's no finalised costing".

Mr Constance said the government would look at building the project in stages because it wanted to minimise disruption to residents and commuters along the likely route.

"We continue to work out the best way to stage the project given we are going down an existing rail corridor between Camellia and Carlingford," he said.

Separate internal government documents obtained by the Herald also showed the project will deliver questionable benefits.

But Mr Constance said: "We are talking about a 22-kilometre network in the greater Parramatta region. In terms of the information today, it's very preliminary."

Labor transport spokeswoman Jodi McKay said the government needed to commit to releasing a business case before work on the Parramatta light rail began, unlike in other projects such as WestConnex "which is being built amid a culture of secrecy".

"Western Sydney and in particular Sydney Olympic Park deserve better transport links. The community has waited long enough," she said.

The government has committed to starting construction of the light rail line before the next state election in 2019.

Western Sydney Leadership Dialogue chairman Christopher Brown said he expected Premier Mike Baird to live up to his "iron-clad promise" to build the light rail line.

"We are no longer willing to cop scraps from the inner-city table. The difference is that the northern suburbs and the eastern suburbs are not paying for their metro line and light rail," he said.

"We are willing to pay for [the Parramatta light rail]. If they are short of money for the Parramatta light rail, put a levy on the obnoxious profiteering in St Ives and Randwick [from the government's metro line and CBD light rail projects]."

He cited a report by consultancy Deloitte last year that found up to $3 billion could be raised from applying value capture to a corridor between Camellia and Strathfield. Value capture is a form of public financing aimed at recovering windfall gains landowners pocket from the construction of public infrastructure.

Mr Brown called for planners to "stop being so damn timid" and demanded the government commit to a new western metro line, the Parramatta light rail project and a north-south rail link via the new Western Sydney Airport at Badgerys Creek.