Former prime minister Jim Bolger told Checkpoint with John Campbell that the Green party might be reflecting on whether they only link themselves to left-wing politics.
Mr Bolger, the former National leader who negotiated the first coalition under MMP with Winston Peters in 1996, said the environment was not party political.
He said the Greens might be quietly reflecting on whether they should only link themselves to left-wing politics
"The environment is neither left wing or right wing, frankly. The environment is the environment, it's Mother Earth we're talking about.
"So I'd imagine in a quiet back room the Greens might be saying, 'Why? Why are we saying we can only go with one party?', eg the Labour party, and you might watch this space if I was you."
Mr Bolger also said a successful negotiation involved trusting and respecting the other side.
"You have to be able to put past differences in the past, leave them in the past and look at what you can do ... because what you're talking about is forming a government for the future, not to re-litigate the past.
"There are some attitudinal barriers that have to be crossed. That's why nobody should expect these negotiations are going to be done in a hurry. The media should relax."
Serious negotiations would not start until the writs were done and the final votes tallied, he said. Official results are declared on October 7.
"It's a matter of careful, patient work and the rebuilding of trust and respect.
"It's only the starting point. What both parties will be doing now is looking at each others portfolios, looking at all the serious statements that were made, seeing where there is a common thread of interest, policy-wise, and seeing where there is really stark differences.
"There's always going to be give and take and compromise.
"It will take some time."
Asked about policy difference in immigration, Mr Bolger said it was about the details.
"There's no perfect number.
"What I think you saw during the campaign is Winston moved out into the provinces and the provincial towns and the farmers in the rural communities are actually wanting more migrants in certain areas, in certain catchments, because there are not enough workers there.
"A lot of people in Auckland are not going to go to provincial New Zealand and milk cows for example.
"So I think the position that Winston was adopting towards the end of the campaign was far more nuanced than some might have thought at the beginning."