John Key, David Cunliffe, Peter Dunne, and Colin Craig were all opposed, though Mr Cunliffe added that it would be a conscience issue, and Mr Dunne said over time he could see a regulatory regime similar to the one for psychoactive substances.
Mr Key said: "Even though I know lots of people use cannabis, in my view encouraging drug use is a step in the wrong direction for New Zealand."
Green's co-leader Metiria Turei said the party wanted favoured decriminalisation, but it was not a priority, while Mana leader Hone Harawira supported cannabis for medicinal purposes.
The poll follows a Herald-Digipoll in June that found majority support for decriminalisation or legalisation of cannabis - though a much higher proportion of voters favoured keeping cannabis illegal.
In that result, just under a third favoured decriminalisation, a fifth wanted legalisation, and 45 per cent supported the status quo.
NZ Drug Foundation executive director Ross Bell said there was a growing acceptance that cannabis had medicinal qualities that could treat issues such as nausea and seizures.
He noted the high-profile case of 7-year-old Charlotte Figi, whose use of cannabis-based products reduced her epileptic seizures.
"Scientists in Colorado, Israel and in the UK are now furiously studying this stuff," Mr Bell said.
"There is a new generation who have grown up with cannabis and probably see it as no big deal.
"This is fundamentally a health issue. There are problems, but education and treatment are a much more effective way of dealing with our most popular drug."
Mr Bell said the absence of political will to change the status quo was "irresponsible".
"There is a point blank refusal on just allowing the conversation, on learning lessons from other countries that have tried alternative approaches. They just don't want to touch it. Even the Greens, who in the past have been staunch, don't want to scare the horses by saying it's not a priority."
In the latest poll, attitudes to keep cannabis illegal were stronger in Auckland than outside Auckland (22 vs 17 per cent), and slightly stronger among men than for women.
The survey, taken between August 14 and 20, had a sample size of 750 and a 3.6 per cent margin of error.
Oddly, those aged 18 to 39 most strongly favoured keeping it illegal (20 per cent), but also had the highest support for making it completely legal (26 per cent), compared to other age groups.
Mr Bell said this result was surprising.
"I don't know how you would explain that. It may well be that a generational shift of people are getting more relaxed, and the flip-side is that people have seen other science that cannabis can be quite harmful to young users."
