Residents in Sydney's south-west around Bankstown and Liverpool will have the next six weeks to nominate where they think new rail stations and train yards should be located for a potential expansion of metro rail to the area.

The state government has raised the possibility of extending Australia's largest public transport project, a metro line from Rouse Hill to Bankstown, from Bankstown to Liverpool.

Transport for NSW has identified a so-called study area between Bankstown and Liverpool in which a metro rail line could be located. The large study area is bordered by the existing Bankstown Line, in the north, and the M5 motorway in the south.

The government has not revealed where it favours stations to be located if, in fact, it does decide to extend the metro line to Liverpool. Neither has it said how many stations are likely to be built or put a price tag on an extension.