A Green MP's suggestion that his party could prefer a second election to a Labour-NZ First coalition is the "height of stupidity", NZ First leader Winston Peters says.

Peters this morning accused the Greens of being arrogant and impractical, but stopped short of ruling them out of any coalition deal.

"We are never going to accept that the Greens care more about this country than I do," he said.

"My family has been here almost 1000 years. And I resent anyone who thinks that I don't care about my country's environmental future."

He made the comments after Green MP Barry Coates said his party would consider forcing another election if Labour formed a coalition with NZ First without the Greens - a statement which earned him a telling-off from the Green leadership.

Speaking to the Herald in Tauranga, Peters said: "I think it's the height of political stupidity to make that sort of threat.

"Because what they're saying is, regardless of how far they fall in the polls - and they will, mark my words - they still want to be in a government and in control of everything."

Peters referred to the untreated timber debacle 15 years ago to criticise the Greens.

"I can recall them demanding the end of wood treatment in this country, even treating posts in the ground because a kid might bite the wood.

"As a consequence, there are tens of thousands of people here who went bust because their homes went rotten. It's that sort of theory and impracticality that NZ First is concerned to eliminate from politics."

The Greens have long rejected the suggestion that they lobbied for the use of untreated timber.

Despite his crticism, Peters refused to rule out working with the Greens in government.

Coates said yesterday that a Labour-NZ First pairing would be "unacceptable" to the Greens and that his party could force another election to prevent it.

He told Newshub the Green caucus had discussed refusing to support a Labour-NZ First coalition in the last fortnight.

Greens co-leader James Shaw said this morning that Coates' comments were "absolutely not" the party's position. Discussions about possible coalition deals were "the reserve of the leadership", he said.

"And we're really committed to providing a stable and responsible government and going the full distance."

Asked whether Coates had been told to "pull his head in", Shaw said: "Yes."

Shaw said the Green caucus had been having "general conversations about the shape of things" after the election, but that did not extend to opposing any coalition which included NZ First.

He said a number of people had expressed concerns to the Greens about the possible influence of NZ First on the next government. The "best insurance" against this was voting for the Green Party, he said.

In a post on the Daily Blog, Coates said that if the Greens were not part of a coalition after the election, they "would not accept a Labour-New Zealand First Government and certainly not a National-New Zealand First Government".

"Neither would be acceptable to the Greens", he said.

Coates referred questions from the Herald to Shaw.

At the Greens' campaign launch on Sunday, co-leader Metiria Turei made similar comments to Coates. But she did not go as far as saying that Greens would force a hung Parliament if Labour and NZ First formed a coalition.

Turei also criticised NZ First, telling TVNZ's Q + A that leader Winston Peters was "on a roll" partly because of "a very racist approach to immigration".

That prompted an angry response from Peters, who appeared to warn of consequences for the Green Party in any post-election talks.

"My warning to the Greens is don't call New Zealand First racist - an allegation that is spurious - and think there won't be consequences."

The Green Party has signed a Memorandum of Understanding with Labour, though that only lasts until election day.

Under current polling, Labour would not be able to form a Government even with the support of both the Greens and NZ First. It would need a fourth party, such as the Maori Party, to get it over the line.

The Greens were locked out of the Labour-led Government in 2005 after NZ First leader Winston Peters refused to work with them.