UPDATED 11.46am: Prime Minister John Key has admitted he underestimated Winston Peters' potential to win the Northland seat at the weekend's byelection.

MORE: No extra MP for NZ First?

The New Zealand First leader took the seat from National at the weekend, trouncing its candidate Mark Osborne by 4000 votes.

Asked on Radio New Zealand this morning how he had underestimated Mr Peters so badly, Mr Key said there were "two things really".

"One was that I thought Labour would run a serious campaign, because they were going to," he said. "I actually saw [Labour leader] Andrew Little at the airport after the launch of their campaign ... essentially he was running a proper campaign, trying to get their candidate over the line.

"Then it became pretty obvious that he just tossed in the towel, and that would swing all those numbers over [to Mr Peters] and we couldn't get there.

"The second thing was I wasn't convinced that Labour-Green voters would actually hold their nose and go out and vote for Winston Peters. But what did happen was they thought, 'hey, here's an opportunity to give the Government a bit of a black eye, so yeah we don't like Winston Peters but the hell with that, we'll go out and vote for him because that will ensure the Government takes a loss'.

"So I got it wrong on that one, but that's the way it goes."

Mr Key said he "always knew it was going to be tough" to win the Northland seat.

"There was always a [New Zealand First] following up there, and in a two horse drag race the guy with 40 years experience vs the guy with 40 hours, that was always going to make it pretty difficult."

But Mr Key said he didn't believe Mr Peters' stealing a Parliamentary seat from National would make a "massive amount" of difference in the Government's ability to legislate.

"We're effectively back to where we were in the 2011-2014 situation, where to pass anything we need Act and United Future, and possibly the Maori Party and any other party that might join us. We can do it with Act and United Future," he said.

However, bringing in changes to the controversial Resource Management Act (RMA) would now be impossible under the current National proposal, he said.

"With the RMA, there's just no question that you've got to rip up what we've got now, go back to the drawing board and have another go."

Mr Key also held firm that National's election trail promises to Northland would go ahead.

"There are things, I won't announce them this morning obviously, but there are things that we are going to announce and going to do," he said. "And we will remain very, very committed there. We'd like to win the seat back."