But there are conditions to any deal being struck with Labour.
Both major parties came in for criticism for being more closely aligned to each other on environmental policies than Green policies.
The decision by the Greens was not surprising given the left-leaning policies of the party, but the announcement would have given Prime Minister Helen Clark some hope that if the polls continued to close, she could cobble together the next government.
The Greens are the only minor party to consistently poll over 5% and would take six MPs back to Parliament on that vote.
National Party leader John Key has already ruled out New Zealand First, and its leader Winston Peters, for any post-election deal but Miss Clark said she was prepared to work with Mr Peters.
National Party environment spokesman Nick Smith said he was disappointed but not surprised the Greens opted to go with Labour.
"New Zealand has been going backwards environmentally for the past nine years. Greenhouse gas emissions have been growing at ever-increasing rates, we have record deforestation and a deterioration in water quality.
"It is extraordinary that the Greens are signing up to more of the same with Labour when co-leader Russel Norman has strongly criticised the Labour Government's poor record," he said.
If the Greens were serious about the environment, they would be prepared to work with either major party to advance important environmental issues, Dr Smith said.
Dr Norman said the Greens had looked at the policies, programmes and public statements of both National and Labour and measured whether their plans would have no effect, or whether they would take New Zealand in the right or wrong direction.
"The Green Party is a strong and independent voice for people and the environment in New Zealand. We don't exist to prop up anyone else's government.
"We found that they are closer to each other than either is to us, and neither of them aligns closely with our own ideals for a fairer and more sustainable New Zealand."
There were individual policies where the Greens agreed with the National Party.
National helped stop a law that would take away control of dietary supplements and the party wanted to see more of the New Zealand Super Fund invested in New Zealand, he said.
However, the analysis showed that, on the whole, National would take New Zealand in the wrong direction, co-leader Jeanette Fitzsimons said.
"In fact, many policies headed off down a dead end street. This means that we cannot form a government with National, or support them on confidence and supply, although we could work with them in areas where we have common ground.
"Labour also seems to be heading off in the wrong direction in some areas, while in others they are on the right track.
"Whether we could form a government with them would depend on reaching a policy agreement that advanced green policies on a number of fronts."
Depending on the outcome of the election, the Greens would prefer to work with Labour to form a government, as its policies were more closely aligned with the Greens.
But, no matter who formed the government, the Greens would look for areas of common ground where the parties could work together, she said.
