Since Associate Health Minister Peter Dunne started imposing temporary bans on synthetic cannabis ingredients 18 months ago, more than 50 products have been pulled from the shelves.
But retailers are still able to sell legal highs by changing ingredients as soon as they are banned.
National Poisons Centre toxicologist Leo Schep said there was a cyclical nature to synthetic drugs.
"[The temporary bans] have been effective, but we will continue to see new analogues come onto the market and then they'll be banned. So we'll see that gap between them, with the arriving and banning; that cycle will very much continue."
Dr Schep said manufacturers of legal highs did not have any concern for the drugs' toxicologies.