A united opposition could have fought the "swing to the right" seen in last night's election results, Green Party co-leader Metiria Turei says.

Labour's mistake was in rejecting a Green proposal to form a united opposition early in the campaign in a bid to leave a door open to work with New Zealand First leader Winston Peters, she said, speaking on TVNZ's Q+A this morning.

"We did put forward to them a proposal of how we could present better as an alternative government earlier this year and they rejected that, I think that was a mistake," she said.

Adding: "One should learn the lesson that you cannot assume what Winston will do by being nice to him. I think that's a lesson that lots of parties learned over time, and it's a lesson that should have been learned already. It's certainly my view, but Labour decided not to present as an alternative government at that stage, [and] we have the result we have."

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Ms Turei said she felt the Green Party was the strongest Opposition in Parliament, and would continue to do so this term. "We certainly believe we led the Opposition last term and we certainly expect to lead the Opposition this term," she said.

"We are highly effective in Opposition which is why we'd be so effective in government, we make no apologies for that at all."

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However, she did not rule out working with National in the new Government, but said the Greens had not forgotten National's refusal to work with them in 2011 and other occasions in the past. "We'll be deciding [our] strategic approach in next few weeks," she said.

But Ms Turei was positive about the party's results, despite polling lower than expected. It had "held on" to its position as the third largest political party, and "still intend[s] to grow".

"I'm not feeling as disappointed as others might think we are, because I know that we have solidified our support and strengthened our vision within a swing to the right."

She admitted that she didn't know why the pre-election polling, which put the Greens at around 14 per cent, did not translate at the booths, but it was something "we will need to do some work to figure out".

Both her and co-leader Russel Norman would "absolutely" stay on as leaders, she said.

"We've got the full backing of our party and our caucus."