John Key is bound for Northland where he will be campaigning alongside candidate Mark Osborne, two days out from the Northland byelection.

The Prime Minister will be fresh off a plane from Japan, and told Radio New Zealand he still thought National could win the byelection.

A 3 News-Reid Research poll last night put Mr Osborne on 34 per cent - well behind NZ First leader Winston Peters on 54 per cent. Three weeks ago, the same poll had Mr Peters on 35 per cent and Mr Osborne on 30. It indicated Labour's nod to its voters to choose Mr Peters had caught on - Labour's candidate Willow Jean Prime had dropped from 16 to 10 per cent.

However, the same poll asked people if they trusted Mr Peters, 48 per cent said no while 43 per cent said they did. Nine per cent did not know.

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Mr Peters told TV3 that the trust factor wasn't influenced by a suggestion he was being "opportunistic" in campaigning for the seat.

"On the 29th I'll be an MP regardless... opportunism doesn't come into it," he said.

Mr Peters said Mr Key arriving in the district to campaign for Mr Osborne wouldn't make a difference.

He said the people in the electorate wanted to know why there had been a "cone of silence" around former MP Mike Sabin's stepping down from the seat.

Mr Key told Radio New Zealand there was no question there were "challenges" in the byelection, but said he still believed Mr Osborne could win.

"We're going to be doing everything we can to win."

He said the byelection could come down to voter turn-out.

"It's in the hands of National voters, because all the other parties are going to have a protest vote against the government."

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He called National the underdog in the byelection, despite the seat usually being a stronghold for the party.