A new poll has NZ First leader Winston Peters holding a commanding lead over National's novice candidate in the Northland by-election.

A 3News-Reid Research showed Peters on 54 per cent, a full 20 percentage points ahead of National's Mark Osborne on 34 per cent.

Earlier polls had Peters, one of New Zealand's most recognisable figures, neck and neck with Osborne, who is a political novice.

Lawrence Smith/ Fairfax NZ BATTLING: Seven Joyce said National was chasing NZ First as the by-election day nears.

The by-election, being held because of the resignation of National's Mike Sabin for personal reasons, will be decided on Saturday.

The poll, which TV3 said sampled 500 Northland electorate voters, had Labour's Willow-Jean Prime on 10 per cent. Labour leader Andrew Little has given a signal to the party's supporters that they could consider voting tactically to support Peters.

NZ First did not stand a candidate in Northland in the 2014 election. According to the poll three-quarters of those backing Peters now had voted Labour last year, with the remaining quarter coming from National.

Just before the poll, National's campaign strategist Steven Joyce said the party was now the underdog. Osborne was "a bit behind" Peters, and had little time to catch up.

Initially National dismissed Peters, with Prime Minister John Key saying the NZ First leader had virtually no change of winning.

But after the early polls showing the candidates were close and Labour's tactical voting signal, National has admitted it is behind.

"I'd certainly say we're certainly in a big battle," Joyce told reporters, before slipping into sporting analogies.

"It's a run chance, and we've got to chase it down and we've only got limited days in which to do it.

"Will we need a six? As I said before, it's definitely challenging. I'd say we're a bit behind, but we're closing."

While Sabin won Northland by 9000 votes in September's election, Joyce said this was a 52 per cent share of the vote, meaning that in a head-to-head race it could be a close battle. Tactical voting and Osborne's low recognition factor were the issues.

"Winston got a big bounce after Labour threw Willow-Jean under the bus, and our man's working very hard to catch up," Joyce said.

"The big difference is Mark's literally been in the market for about three weeks, and Winston has been around for 40 years."

National chose to have the by-election two weeks earlier than it could have.

Joyce said that he did not make the decision on timing, and it was not known that Peters would be standing.

"The party and the Government set the timing. As the campaign guys you work with the timing. It should be enough time, but it's tight and that's the point, so we're going flat out," Joyce said.

"It's been a tougher challenge than we expected, there's no doubt about that."