Southern mayors who campaigned for a ban on legal highs say the country should not now be considering decriminalising marijuana.

Auckland deputy mayor Penny Hulse is urging the city's council and the Government to hold a discussion on whether cannabis should be legalised.

The Invercargill, Southland and Gore mayors had lobbied to have all synthetic highs banned.

Last week, legislation came into effect making it an offence to sell, supply or possess synthetic cannabis and other psychoactive substances. The products will be allowed back on shelves only after being proven to be safe, without testing on animals.

Southland District mayor Gary Tong said legalising cannabis was not something he would consider, given the damage he has seen all drugs do to the community and to families.

Gore District Mayor Tracy Hicks said there were long-term challenges that go with prolonged use of cannabis and he does not agree with decriminalisation.

But he said the use of cannabis for medical reasons was something worth discussing.

Photo: RNZ

Penny Hulse, who also chairs the Legal Highs Working Party, says until a few weeks ago she firmly believed cannabis should be illegal, but the ban on synthetic cannabis had driven its sale and use underground, making it unsafe.

She said said cannabis was still harmful but may be the less toxic choice when compared to its synthetic alternative. Legalising the natural form of cannabis could be part of a solution for dealing with drug prevention, she said.

"When you look at the comparison between organic marijuana and synthesised legal highs, all the scientific evidence I've read leads me to believe that cannabis, although damaging and concerning, is a less toxic choice," she said.

"All the advice, in particular from Professor Ian Shaw, the toxicologist from Canterbury University, the impact synthetic cannabis has on the brain is far more damaging than that from cannabanoids."

She said the science should stand up in discussions about cannabis legalisation.

Ms Hulse said the discussion as to whether it was banned, legalised, or decriminalised, needed to be had, and not used as a political football.

She said the work put into legalising cannabis would be similar to that put into developing policies for legal highs management.

Gone underground

Ms Hulse said she was worried the council could no longer reach some of the young people using synthetic highs and stop them taking the drugs in the first place, or support them to come off them.

"We're really concerned that this has now pushed the entire issue underground, for the time being," she said. "At least we had a window of trying to work with our community to prevent drug use ... we now don't have that opportunity."

She said the Government was on the right track trying to manage the legal highs industry.

"The bit we're missing now is people think the issue has disappeared, it certainly hasn't, it's gone underground," she said.

"People will continue to find ways of finding something to get them stoned."

Ms Hulse said the issue needed to be brought back into the daylight, and the community could have a reasoned conversation about how to deal with it.