Labour's leadership contest has given the party a slight boost, widening the lead an alliance on the Left would have on National.
The latest Roy Morgan poll puts Labour on 32.5 per cent (up 1 percentage point from last month), with the Greens also rising to 15 per cent. This would put the two parties on 47.5 per cent, holding its biggest lead since April.
National has fallen three percentage points to 41 per cent, and support for its coalition partners has barely moved.
The pollster's "confidence rating" is also at its lowest level since April, down 10.5 points. More than half (52.5 per cent) of those polls said New Zealand is heading in the right direction, 35.5 per cent plumped for the wrong direction.
Labour's new leader is set to be elected on Sunday after a two-week hustings "roadshow".
Acting leader Grant Robertson, New Lynn MP David Cunliffe and list MP Shane Jones are all vying for the top job after David Shearer quit last month.
Roy Morgan Research executive chairman Gary Morgan said a Labour-Green alliance would "win easily."
"Whoever emerges as the new Opposition leader on Sunday stands a good chance of becoming New Zealand's next prime minister as the head of a Left-leaning alliance," he said.
The Maori Party fell by 1 percentage point to 1 percent, ACT were up 0.5 percentage points to 1 percent.
UnitedFuture was unchanged on 0.5 per cent.
The Conservatives - often touted as a future National ally - rose 0.5 to 1.5 per cent.
NZ First got a slight lift of 1 percentage point to 6.5 per cent and the Mana Party was unchanged on 0.5 per cent.
Is it time for Judith Collins to go?Related story: Judith Collins looking isolated