Green MP Barry Coates says the party would refuse to support a Labour-NZ First Government - and even indicated the Greens would be prepared to force another election to stop it.
In an extraordinary move, Mr Coates has said a Labour-NZ First Government would be "unacceptable" to the Greens.
Asked by Newshub if this meant forcing the country back to the polls for another election, Mr Coates responded: "It could do."
Mr Coates also said Green MPs had discussed refusing to support a Labour-NZ First combination as a caucus in the past fortnight.
"[Labour-NZ First] could not count on the support of the Green Party there is no automatic support that is provided for a Government under those circumstances," Mr Coates said.
Mr Coates was responding to a post on The Daily Blog where he said: "The memorandum of understanding (MOU) with Labour is the foundation for building the next Government. If we were not part of the coalition, we would not accept a Labour-New Zealand First Government and certainly not a National-New Zealand First Government. Neither will be acceptable to the Greens."
In such a scenario, where the Greens held the balance of power, neither side would have the numbers to govern and the only option would be to force the country back to the polls - costing the taxpayer millions of dollars - or Winston Peters could give his support to a National-NZ First Government.
The scenario is possible as NZ First leader Winston Peters is likely to ask to be the "second cab off the rank" in a Labour-NZ First Government, with the Greens supporting it from outside - as he did in 2005.
All Newshub-Reid Research polls this year show Labour, the Greens and NZ First would have to work together to beat a National-led Government, and the Labour-Green MOU to work together expires on election day.
Mr Coates' remarks come after a week of deep division between the Greens and NZ First, that started with co-leader Metiria Turei calling Winston Peters racist.
In her speech to the Greens' campaign launch in Nelson she said much the same as her MP.
"If the Greens are not in government after September, our country will face either a Labour-New Zealand First or a National-New Zealand First government. Neither is acceptable to the Greens."
He followed this up with a plea for votes.
"We must face all of our efforts between now and September 23 on preventing those two scenarios. The strongest possible party vote for the Greens is the only solution."
Greens co-leader James Shaw was last night forced into damage control about Mr Coates' remarks - saying Mr Coates had the wrong end of the stick.
"What Metiria was saying is if you are worried about that as a voter then the best thing you can do is give your vote to the Green party."
Mr Shaw wouldn't discuss what the party's options were should the scenario play out the way Mr Coates has painted it.
"Frankly I think that there's a lot of scenarios that could play out at this election and we just think everything is hypothetical until you know how many MPs each party has got."
"I don't think it's particularly helpful to chuck round lots of different scenarios because there are actually lots of different scenarios.
"Look there's a lot of scenarios I don't want to get into what all of the hypothetical situations are.
"Obviously it would be very difficult for anyone to form a Government without us."
But Mr Shaw said the Greens would not hold the country to ransom.
"That's not our style."
But Mr Coates was clear in his interview with Newshub, saying: "Well you know in 2005 NZ First and Labour formed a Government without the Greens. We're signalling that our support should not be taken for granted that we would automatically support a Government with Confidence and Supply under that scenario."
And Ms Turei did allude to negotiating hard when it comes to post election talks.
"NZ First practices a divisive form of politics that sets New Zealanders against each other. Only a party vote for the Greens will weaken NZ First in a Labour-led Government. We will support a truly progressive Government, and NZ First may play a part in the government, but we will not accept an inferior deal."
Mr Coates added: "There are various ways of not necessarily going back to the polls - not providing confidence and supply for example, sitting on the crossbenches - there's a lot of different scenarios."
However, those options would still allow a Labour-NZ First Government or National-led Government to be formed.