A binding referendum on legalising cannabis for personal use will be held at the 2020 general election.
In an editorial in the NZ Medical Journal, Dr Joseph Boden, of the university's Department of Psychological Medicine, said before any decision was made on legalisation, the drug should be decriminalised, and the results of that assessed.
"If this evaluation showed that the decriminalisation process reduces harms, the next stage would be to move towards further liberalisation of cannabis laws," he said in editorial co-authored with the late Professor David Fergusson.
Dr Boden told Summer Report if a drug was legalised it would be hard to reverse if evidence showed the change had caused harm.
There was little evidence in US research of increased use among young people, but both cannabis use and disorders had risen among adults.
"People who previously had used only occasionally, or who had tried it and given up, are taking it up again and those who used not very frequently are using more often. That's the particular area of expansion that we worry about."
He said cannabis dependence was an under-recognised disorder and affected roughly 15 percent of the population at some point in their lives.
"One of the areas that isn't well recognised is cannabis withdrawal which can have quite severe cognitive side affects - confusion, mental slowness, irritability - that is not very well recognised amongst physicians."
But Green Party spokesperson for drug law reform Chloe Swarbrick said decriminalisation did not go far enough to reduce harm.
"Under a decriminalised model people are still going to need to get access through the black market and we're not going to be wiping out gangs."
Ms Swarbrick said the best way to regulate and to minimise harm was to improve education and to ensure young people are not getting access.
"If we were to solely decriminalise this substance, we would not see that reduction in young people having access because, quite frankly, drug dealers are not checking ID."
Dr Boden proposes decriminalisation of possession of small amounts for those over 18, increased penalties for supply to those under 18 and more investment in mental health services for cannabis use disorder and cannabis-related conditions.
He welcomed the approach of treating drug use as a health issue.
"There's absolutely no upside in criminalising people who are drug users - none at all," he said.