The Government has left voters in the dark ahead of this year's cannabis referendum, according to New Zealand's leading drug academic.
Chris Wilkins says we're lagging behind the likes of Australia and Canada when it comes to measuring the effect of drugs on society.
But the Government says we do know enough to make an informed decision about whether or not to legalise cannabis.
"We're a little bit in the dark, so we know a certain percentage of the population are using cannabis, but we don't know how many of those people are really heavy users," Mr Wilkins told 1 NEWS.
The Massey University researcher is challenging the Government, saying they've failed to collect enough information about cannabis use ahead of this year's referendum.
He says countries like Australia and Canada run regular detailed research surveys, yielding greater insight and understanding into how drugs are used, while we only ask a few questions once every two years in a much wider national health survey.
"Really, we're basing that understanding on a lot of overseas jurisdictions, who are often fundamentally different from New Zealand," he says.
The national health survey does give a big overview, showing at last count around 590,000 New Zealanders had used cannabis in the 12 months prior. However, there's no further detail.
Justice Minister Andrew Little argues that number, along with data from the agencies like police, does give voters enough to make up their mind.
"We're doing it with the information we've got," he told 1 NEWS. "I think most people know that cannabis is a substance that if you use it has an affect on your brain.
"Regardless of the numbers of people it could be affecting, knowing what it can do, what we do we to control it and regulate it, minimise the harm."
Mr Wilkins is urging the Government to boost research.
"That work has to start pretty much now if we're going to have any kind of baseline data before the referendum happens."
If it's a 'yes' vote come the election, time will tell whether the Government does have enough information to make good policy.