Updated 3:40pm: Social policy advocates are calling it a case of deja vu - rushing through an ban on legal highs without giving time to help drug addicts.
The ban on the sale of untested psychoactive substances comes into effect at midnight.
NZ Drug Foundation's Ross Bell says synthetic drug users need time to stop using the illegal products.
He says it happened when BZP was reclassified as a Class C drug, only the 2008 law change had a six month grace period.
"This amnesty should have been built into the law but what we need now is for the police to have a policy of non-prosecution for the next six months.
"We really do need to make sure that we remove any barriers to people who might need help for these products."
Mr Bell says this week's legislation outlawing untested psychoactive substances from midnight was rushed and risks dire consequences.
"There's a real risk now that they'll be too fearful to look for help, they'll be fearful of the fact that they might get busted, the cops will pick them up, they might get a criminal record, and therefor not go looking for that help."
Ban will increase harm
Industry claims that the ban on synthetic highs will only increase harm.
Grant Hall of the STAR Trust, which represents the industry's interests, told Mike Hosking the market for these products exists.
He says the ban will only drive it underground.
"Where no one checks ID, no one does quality control and no one pays taxes, so inevitably harm will increase."
Counselling services concerned
Addiction counselling services are also concerned about what will happen when legal highs are banned.
Salvation Army spokesperson Alistair Herring says there are worrying signs about what substance abusers are doing with the law change.
"There is clear evidence that there is stockpiling happening.
"This is going to be a worry for those who are doing the stockpiling, the sale going underground on the black market is a matter fore the police but is also a community concern."