The Green Party captured 10.1 percent in the latest DigiPoll, the party's highest rating in the poll since the 2002 election campaign.
With Labour floundering around the 30 percent mark in most polls, and no other minor party polling higher than 2 percent, the Greens could hold the balance of power should National fail to win 50 percent of the vote.
The Greens have not ruled out working with National, but it's has been described as "unlikely" by the party's leaders.
This morning John Key National leader told TV One's Breakfast the Green Party are a natural partner for Labour.
"I think it is important to understand if the Greens hold the balance of power it would be a Phil Goff Labour-led government and I think they would be quite upfront about that," Mr Key said.
"They are socially quite a long way to left and economically they want to put a lot of costs on businesses and slow down the economy so in that regard they are much more akin to Labour."
Meanwhile Key has not ruled out endorsing Act's Epsom candidate John Banks.
Banks is polling below National's Paul Goldsmith, and without the seat Act will almost certainly not be returned to Parliament, meaning National lose a natural ally of their own.
"All I can say is the last three years people got the chance to assess how we work and I think we have worked very constructively with the Maori Party, Act and United Future," Mr Key said.
"And Act has been very stable so Act returning to Parliament is something I would like to see as opposed to something I wouldn't like to see."
- Paul Harper
