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.
"It really is a lottery with what these kids are taking."
He said the number of analogues of these sorts of drugs meant it was hard to keep on top of them.
However, when the law comes into effect later this year, Dr Schep expects a stall in the industry.
Mr Dunne said "temporary class drug notices" were always a holding pen until the Government could bring in permanent legislation reversing the onus of proof, so the industry will have to prove any product is safe before it can be brought to market.
That regime would be in place by the middle of this year, he said.
"In saying that, the temporary class drug notices have been a very successful holding pen, with 32 substances now removed, and therefore effectively more than 50 products that have been taken off the market."