The capacity of the western rail line between Sydney's CBD and Parramatta to handle more commuters will be exhausted within 15 years, Transport Minister Andrew Constance says.

The Herald reported last week that government planning is under way on a new metro line between the central city and Parramatta to bring much-needed relief to Sydney's western rail corridor and service growth areas such as the Bays Precinct around Rozelle.

Speaking at an Urban Taskforce function on Wednesday, Mr Constance said there was "no doubt that there is going to be the need for additional rail capacity" between Parramatta and the city in the longer term.

"[However,] the government has made no decision about western metro – let's be clear on that. There is some private sector interest out there, absolutely," he said.