The National Government says the Labour and Green parties are in disarray, with starkly different policies on deep sea oil drilling.
But Labour rejects that and says the Government is being mischievous.
The Greens want to immediately ban oil drilling, whereas Labour supports it, though with tighter regulations.
Economic Development Minister Steven Joyce said the potential Labour-Greens coalition should develop joint policy positions.
"Labour and the Greens are - officially, anyway - quite far apart on some of these policies," he said.
"What you would end up with post-September 20 if a coalition turned up, is they'd be trying to work it out then, and that'd be a recipe for stagnation and confusion."
But Labour's energy spokesperson David Shearer said all parties would have to negotiate various policy areas after the election. That's the nature of MMP, he said.
"He knows parties will have to get together; they'll have to work through policies together. So will National. They'll be needing to go together with perhaps the Conservatives. That will create Mr Joyce some problems."