The presence of Prime Minister John Key in Northland has nothing to do with New Zealand First Leader Winston Peters' taking an early lead in the race for the Northland electorate byelection, Mr Key says.

A 3News/Reid Research poll late last week gave Mr Peters 35 per cent support over National Party candidate Mark Osborne's 30 per cent and Labour's Willow-Jean Prime on 16 per cent in the Northland electorate race.

On Saturday Mr Key hit Topuni, Kaiwaka, Mangawhai and Mangawhai Heads in a public show of support for Mr Osborne.

He will make two more visits to Northland to support Mr Osborne before the March 28 byelection, which will cost taxpayers up to $1million.

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Mr Key's visit to the Mangawhai Village Market on Saturday was preceded by Mr Peters also doing the rounds of the market before the PM and his entourage arrived.

At Mangawhai the Prime Minister called Mr Peters the "fly in the ointment" for National's chances of regaining the seat it has held since the 1960s, predicting opposition voters would flock to him. He said the visit had nothing to do with Mr Peters' poll showing.

If Mr Peters did win the byelection it would not overturn the Government, but it would leave National with 59 seats in the 121-seat Parliament, meaning it would require two votes from a combination of the Maori Party's two seats or the single votes of Act and United Future. The byelection has arisen after National MP Mike Sabin resigned for personal reasons.

Meanwhile, Mr Peters said he would be making a "crucial policy for the infrastructural future of Northland" in Whangarei today.

Also standing in the byelection are Adrian Bonner (Independent), Joe Carr (Focus New Zealand), Robin Grieve (Act Party), Maki Herbert (Aotearoa Legalise Cannabis Party), Adam Holland (Independent), Rob Painting (Climate Party), Rueben Taipari Porter (Mana Movement), Willow-Jean Prime (Labour Party) and Bruce Rogan (Independent).

Advance voting opens on Wednesday.