Winston Peters says the Northport rail project at Marsden Point is his bottom line for any coalition deal.
NZ First has been strongly advocating the connection, which may cost up to $1 billion.
Mr Peters says it's the first thing both National and Labour will have to concede if he's the kingmaker.
"I can say for the people of Northland and Whangerei this is going to happen," he told The Nation on Saturday morning.
"We've got the corridor, it's been designated - the only thing it lacks is the commitment from central Government, and that's one of the first things we're going to be doing straight after the election."
Mr Peters added he will fire the board of Kiwirail for incompetence.
KiwiRail says the cost to get the Northland rail network operating the same as other regions is somewhere between $700 million to $1 billion.