A ban on legal highs is not possible because the chemical composition of synthetic cannabis keeps changing, Associate Health Minister Peter Dunne says.
But the National MP for Northland Mike Sabin said the Psychoactive Substances Act would end up being a ban "by default" as it would be too expensive for people to get new products to pass the act's strict threshold.
Mr Dunne was largely behind the act, introduced in July last year, which allowed licensed retailers to sell psychoactive substances - also known as legal highs - deemed to pose a low risk of harm.
Under the act, local authorities can set up a local approved products policy that can limit where the substances can be sold - but they cannot ban the products, even if they want to.
Whangarei District Council is seeking public feedback on its proposal to limit the sale of psychoactive substances to a three-block area of the CBD, but many people want them outlawed.
The council plans to restrict where legal highs can be sold in the central city to Lower Cameron St, Clyde St and Albert St and will only allow legal highs to be sold between 10am and 2pm, 6pm and 8pm. Legal high stores could not be within 300m of each other, meaning a maximum of two to three stores in the zone.
Mr Dunne said the Government was unable to ban legal highs because the chemical composition of synthetic cannabis keeps changing and if it banned a product it would be replaced within days by a different chemical compound.
A spokesman for Mr Dunne said Ireland banned legal highs in May 2010 and research there showed the ban had driven the problem underground, with no controls over the products.
"Their Minister in talks with Mr Dunne, said the problem has not gone away ... the world is now looking at New Zealand to see if what we are doing [with legal highs] will work elsewhere," the spokesman said.
He said the Government felt the act was the best way to restrict legal highs and when the second phase comes in in 2015 it could prove too expensive for people to produce substances that met the act's conditions.
Once the second phase came in, all current licensed products - 41 at present - will have to be withdrawn and new products would have to be deemed safe under the act.
Mr Sabin said this would cost $1 million to $2 million and take up to two years, something that would prove too difficult to do.
"They will have to prove there is no harm or risk of harm from the product. That will put restrictive costs on the product that in the end will be of no benefit to the customer in terms of a high."