The mayors of Southland are demanding the Government ban the sale of synthetic cannabis and other legal highs.
The Psychoactive Substances Act came into force last year, and requires manufacturers to prove legal highs are safe before they sell them.
Southland district mayor Gary Tong said he, along with and the mayors of Invercargill and Gore are questioning why the Government introduced what they regard as a weak law.
He said the communities of Southland have sent the message they want a total ban on legal highs and he disagrees with the Government's position it cannot achieve that.
Mr Tong said the law needs to be changed to prevent any more people from becoming addicted.
But Associate Health Minister Peter Dunne said the mayors do not understand the issue.
Mr Dunne said no-one in the world has yet devised a solution that means the substances, which are constantly being developed, can be banned.
He said councils asked Parliament for the power to have local controls on the sale of legal highs and the new law provides that.
Mr Dunne said the new approach to controlling legal highs has already resulted in a reduction of their sale.
Drug Foundation director Ross Bell also said the new law is already working.