The state government should move fast to lock in plans for another metro rail line to run through Parramatta, Olympic Park, Rozelle and the city to reduce the cost of building stations, business representative say.

Fairfax Media revealed on Thursday that planning was well underway inside Transport for NSW for a western metro project to run between Sydney's central business district and Parramatta.

But the project could come at the expense of sections of a planned light rail network around Parramatta, construction of which was due to start in 2018.

David Borger, the western Sydney director of the Sydney Business Chamber, said the mooted metro line would be great for the area. But Mr Borger cautioned that it would be best to decide on station locations earlier.