To be funded under the metropolitan transport plan, Ms Keneally said the largest single pledge of the federal election campaign had been the announcement the federal government would contribute $2.1billion towards the $2.6billion rail line.

The proposal would see the link built by 2017, but the state Opposition has questioned how NSW would find the money given that there were no spare funds in the $50billion metropolitan transport plan.

The Opposition's transport spokeswoman, Gladys Berejiklian, claimed the announcement had been hatched ``on the back of a ballot paper''.

She also said that documents obtained by the state Opposition showed the claim by Prime Minister Julia Gillard that the project was ``ready to go'' was wrong.

The documents showed, among other things, that there were no patronage figures, cost estimates, environmental or geotechnical studies completed before the commitment was announced.

But the Premier said both Treasury and the budget committee of cabinet had signed off on the deal before it was announced.

``That $500million that we are committing to the Parramatta to Epping rail link represents one per cent of a $50billion, 10-year plan,'' Ms Keneally said.

``Before we accepted the commonwealth's offer to enter into an 80-20 partnership ... we put this through a budget committee process, we sought advice from Treasury. Treasury gave very clear advice that this could be easily accommodated.''