Executive director Ross Bell said the fear of getting caught was one of the biggest barriers to people seeking treatment.
Official advice showed an estimated 150 to 200 people were believed to have developed dependency on synthetic drugs and their withdrawal would need to be managed.
A grace period was used when BZP was reclassified as a Class C drug in 2008.
Mr Ryall yesterday ruled out a police amnesty. "The law is clear and it's up to police and how they deal with the ... enforcement provisions."
Police said they would not adopt punitive measures at first.
Inspector Rob Duindam of police national headquarters said officers would take a "common sense" approach to possession cases.
"The important thing at the moment is that we focus on minimising the harm to users by ensuring that the products are removed from sale in an orderly way and that retailers comply."
Possession of a small amount of synthetic cannabis or party pills could lead to a $500 fine, but manufacturers or retailers who flout the law risked heavier fines or prison.
Ministry of Health officials have warned the Government it was very difficult to predict the impacts of the law change because of the rush to put the ban in place.
"The pressure of time has not allowed us to confidently measure the impacts of this proposal on people who use them, on industry and on government and community agencies," a ministry paper said.
During the debate on the emergency bill, Green MP Kevin Hague questioned why the Government had taken drastic action when health authorities had reported a fall in adverse reactions.
The Psychoactive Substances Regulatory Authority has reported that the number of severe presentations to hospital emergency departments has reduced since the Psychoactive Substances Act came into force in August.
Mr Ryall responded: "If you're saying to me was there some sort of scientific system where every person presented to any health service was being added up, there's no information on that basis."
But he said reports from the National Poisons Centre and district health boards led the Government to believe the products with interim waivers were not low-risk. The Government already had powers to order individual products from the market, and had previously ordered six brands to be off shelves by April 30.
Illegal highs
• All synthetic products now banned until a testing regime is in place
• 148 retailers have lost licences to sell synthetics.
• 35 brands have been pulled from shelves.
• Penalties include a $500 fine for possession, or $500,000 for manufacture or supply.
• 150-200 people expected to require treatment for withdrawal.
• Estimated losses of $140 million a year to industry while ban is in place.
