Under proposed legislation importers of psychoactive substances will have to prove they have legitimate uses.

The reform is aimed at closing a loophole that allows synthetic drugs to avoid prohibition by a slight change in their chemical make up.

This is because existing criminal laws ban illicit drugs based on their chemical structure.

In Tasmania an initial rise in synthetic drug use appears to have tapered off.

Last last year Tasmania Police seized a number of drugs with names like Marley, Malibu, Green Lantern, White Devil and WRX.

However, overall they seized less than half the amount of synthetic drugs in July to April than what was nabbed the previous period.

The former state government had banned a number of synthetic cannabis derivatives, but was awaiting a report from the intergovernmental committee on drugs about the issue before considering a full ban.

Federal Justice Minister Michael Keenan yesterday said the burden of proof would be reversed so importers would have to justify the substances.

"This means governments and law enforcement are not trying to play catch up every time a new synthetic drug is produced," he said.

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