Large, highly-publicised drug seizures are not reducing drug crime or drug-related harm, a study suggests.

The New South Wales Bureau of Crime Statistics study, which examined the effects of large drug raids and supplier arrests over the decade to 2011, is believed to be the largest project of its kind undertaken in Australia.

Bureau director Don Weatherburn said it found the raids had "no effect on crime, no effect on overdoses [and] no effect on use and possession of drugs".

"Whenever you see police with a table full of some huge seizure of cocaine amphetamines or heroin you often hear it said that those are drugs that didn't get on the street and didn't cause anyone to die or suffer," Dr Weatherburn said.

However, he said, police were finding drugs more frequently because more drugs were coming into the country.

"The only effects we found ... was a tendency for when the seizures went up, for overdoses to go up, and for arrests, for use and possession to go up," Dr Weatherburn said.

He said that was an indication that "when there is a big seizure that you have actually got more drugs coming in, rather than fewer drugs getting to the street".

The study, which was funded by the National Drug Law Enforcement Research Fund, concluded that in the months following large seizures there was no reduction in the use of heroin, cocaine or amphetamines such as speed or ice.

However, it found several major anti-cocaine operations in 2010, which seized almost 700kg of cocaine in three raids, did have a positive effect.

"They were the only three which we saw produce a reduction in overdoses and a reduction in arrests for use and possession, but it was only temporary," Dr Weatherburn said.

Dr Weatherburn said the results did not mean drug raids were a waste of time.

"All we have done is rule out any short-term association between drug seizures and drug-related harm," he said.

"What is more important than the quantity of drugs seized is the fear and loathing that you create amongst drug traffickers of being caught and being put in prison.

"They won't get involved unless the profits are huge and that ends up being passed to drug consumers in the form of higher prices and that pushes down consumption.

"To put it plainly it is better to catch a half a dozen drug traffickers, each of whom has a kilo, than to catch one with five kilos."