ALL psychoactive substances such as synthetic drugs will be prohibited unless importers can prove they have a legitimate use, under new legislation to be introduced to Federal Parliament.

Federal Justice Minister Michael Keenan said the radical overhaul would close a loophole that allowed people to deliberately avoid prosecution by slightly changing the chemical structure of a banned substance so it could be legally imported into the country.

"Synthetic drugs are often marketed as safe and credible alternatives to illicit drugs yet, tragically, we have learnt too many times through death or injury to people - especially young people - that this is not the case and they are extremely dangerous," Mr Keenan said.

"There's nothing synthetic about the damage these drugs cause.

"They are not harmless, they are not safe, they have deadly consequences."

Mr Keenan said existing criminal laws banned illicit drugs based on their chemical structure.

But the rate at which new versions appeared made it hard for laws to keep up.

"Rather than law enforcement having to prove a synthetic drug is illegal, the burden will be on the person importing it to prove that it is legal and has a legitimate use," he said. "This means governments and law enforcement are not trying to play catch up every time a 'new' synthetic drug is produced.

"The new legislation will target psychoactive drugs marketed as 'legal' alternatives to illicit drugs like ecstasy, LSD or cannabis."

The ban will not replace existing illicit drug offences, which will continue to be the primary way law-enforcement officers deal with illicit drugs and the people who try to import them.