An internet panel poll that put New Zealand First in the running to return to Parliament and took a chunk off National's support is not credible, Prime Minister John Key says.

The Horizon Poll surveyed 2000 voters in November. People can sign up for Horizon polls by joining a website, and can win prizes for participating.

The poll put New Zealand First support at 6 per cent - support which if replicated on election day would be enough to see its leader Winston Peters return to Parliament, and play a role in deciding which party governed.

NZ First won just over 4 per cent of the vote in the 2008 election and Mr Peters lost his Tauranga seat, so the party did not make it back into Parliament.

The poll put National on 34.7 per cent, Labour on 28.3 and the Greens at 7.9.

Mr Key told Newstalk ZB the poll was not credible.

"I just don't accept the poll to be perfectly honest," he said.

"If you look at every public polling company...they haven't had a poll where National's been under 48 per cent for two years, actually I don't think they've had a poll where we've been under 40 per cent for the entire time I've been either leader of the opposition or prime minister."

He told Breakfast on TVNZ that internet polls were unreliable.

"Internet polls are capable of doing anything. I think it's great place to sell newspapers, I don't it's necessarily a good thing for political analysis."

Before the last election Mr Key ruled out forming a support arrangement with Mr Peters' party. He hasn't repeated that stance - yet.

"It's very hard to take Winston Peters seriously... I will worry about it if he decides to throw his hat in the ring, I will make the call then."

Curia Market Research Ltd director David Farrar said internet panel polls were more reliable than website polls, but generally less reliable than phone polls.

"Companies recruit people to join the panel, but people can also just sign up on their website etc, so it is partially self selecting," he said.

"Some of the larger overseas polling companies have such huge panels - over 100,000 - that they can effectively pull out random samples from within their panel, say ask 5000 of the 100,000 to take part, which tends to make them reasonably reliable."

He said in New Zealand panels were quite small so companies tended to use the entire panel for their surveys which made them less reliable.

- NZPA