The Labour Party and the Green Party have confirmed that they will work together at next year's election in a bid to change the Government.
Labour leader Andrew Little and Green co-leaders James Shaw and Metiria Turei announced this afternoon that they had signed a Memorandum of Understanding.
"It is time for a change," Mr Little said.
"Labour and the Greens have reached an agreement, common ground, that now is the time to start the work to change the Government in 2017."
New Zealanders would now be in no doubt about what an alternative Government would look like, he said.
Mr Little said that if the Labour-Greens coalition came into power, Labour's finance spokesman Grant Robertson would be finance minister. No other portfolios had been discussed.
Ms Turei said the partnership would mean it was "crystal clear" what a new Government could look like. The two parties had distinct qualities, she said, but shared a vision to bring about "inspiring change" in New Zealand.
"We are a great match for creating a better future for Aotearoa New Zealand," she said.
The formal partnership is a significant step. Labour rejected an approach by the Greens to work together in 2014 - a decision which former Labour leader David Cunliffe later said was a mistake.
The partnership would have implications for any potential coalition deal with New Zealand First, who have previously ruled out working with the Greens.
Ms Turei said the Memorandum of Understanding did not eliminate potential coalitions with other parties, including New Zealand First.
Mr Peters refused to say this afternoon whether the Labour-Greens MoU would mean New Zealand First could not go into Government with the two parties.
He would not directly answer questions about whether he could work with the Greens.
Mr Peters said that MoUs "had never been the mantra of New Zealand First".
"We do not like jack-ups or rigged arrangements behind the peoples' back. We'll go into the election just ourselves and our policies seeking to change how this country is governed."
National Party campaign manager Steven Joyce said the two parties' MoU appeared to have a number of "weasel words" and out-clauses.
"Labour couldn't quite say whether they'd prefer the Greens over Winston Peters or Winston Peters over the Greens," he said.
"So that's where is started to sound like an episode of The Bachelor to me - they couldn't quite make up their mind."
Mr Joyce said the joint partnership was unlikely to affect National's campaign strategy.
It would allow National to re-use last election's TV advertisement, he said, which showed Labour and Greens in a boat going nowhere.