Prime Minister John Key is to cut short his state visit to Japan to campaign in the Northland by-election.

On Monday afternoon Key announced that next week he will travel to South Korea to witness the signing of New Zealand's free trade agreement, and then on to Japan for talks with Prime Minister Shinzo Abe.

But he admitted that the trip was shorter than initially planned to allow him to have a final push in Northland ahead of the March 28 vote.

Northland, which has been held by National for more than half a century, is heading to the polls after MP Mike Sabin quit for personal reasons earlier this year.

Although Sabin won the electorate by 9000 votes in September's election, polls have shown National's candidate Mark Osborne and NZ First leader Winston Peters are neck and neck.

National has reacted by sending wave after wave of MPs and ministers to the electorate.

Conservation Minister Maggie Barry visited the electorate on Saturday, Economic Development Minister Steven Joyce visited on Monday (in place of Deputy Prime Minister Bill English) and Energy Minister Simon Bridges is due to travel to Northland on Wednesday.

Along with the ministerial visits, a number of backbench MPs have also travelled to Northland, appearing to provide assistance for Osborne on the road. A Beehive staffer from Local Government Minister Paula Bennett's office is on leave providing assistance full time.

Key said last month that Peters had virtually no chance of winning the by-election, but on Monday he said that a combination of low turnout, and the difficulties incumbent governments faced meant it would inevitably be close.

"Even in our safest seats, by-elections are very fickle, difficult matters, there's no getting away from that."

Confirming that he would cut short his trip to Japan for the end of the campaign, Key said he would be criticised whatever he did.

"If I didn't come back people would say I didn't care about the people of Northland, if I do come back people will say I'm worrying too much about Northland...It's an important by-election for us, we'd like to hold the seat if we can."

He dismissed questions about whether the resource National was throwing into the by-election - during a period when Parliament is sitting - saying all travel was within the rules and Peters had been absent from Parliament all of last week.

"The rules are really, really clear. A minister is free to use both air travel and ground travel any time they want, and in the middle of an election campaign, I'd use my ministerial limousine," Key said.

"MPs are free to fly any time they want, there are no restrictions and Parliamentary purposes required."

Key said he understood National's MPs were either being billeted or not staying in Northland, and so no accommodation costs were being charged to the taxpayer.