The offer included $250 million of the city's money and a Government commitment to pay for half of the project from 2020.
Mr Twyford said Labour would pay the Government's share up front, indicating the money would come from reviewing the Government's roads of national significance, including the so-called "holiday highway" from Puhoi to Wellsford.
He said Labour would spend up to $400 million on improvements to the existing stretch of State Highway One between Puhoi and Wellsford, including a bypass of Warkworth and straightening out and improvements to tackle the accident black spots.
"We do intend to make the rail link an election issue," said Mr Twyford, who said Mr Key's "flip-flop" last year in coming round to the project indicated he was feeling the heat politically.
Mr Twyford said Mr Key would be a vague memory by the time National delivered on its timeframe to start building the rail link in 2020.
Last week, Mr Key poured cold water on Mr Brown's offer of an early start, saying officials would look at it "but at this point really, nothing has changed".