Western Australia is to outlaw synthetic cannabis from Friday. It is the first state in Australia to do so.
But the state opposition has criticised the government for not imposing an immediate ban.
The ABC reports that government agencies have been investigating into the effects of products that mimic cannabis.
Mental Health Minister Helen Morton says there is enough evidence the substance harms people who smoke to warrant the ban.
Ms Morton says anyone caught with the drugs from Friday, when the ban goes into effect, will face severe penalties.
''If people have stocked up and have a certain amount of this product on hand, that will be considered (as) possession with intent to sell or supply,'' she said.
''The penalty for that is up to $A100,000 and or 25 years' jail.''
Synthetic cannabinoids are contained in products such as kronic, kalma, voodoo, kaos and mango kush, which are sold in a number of shops.
However, Opposition health spokesman Roger Cook says the decision to allow the sale of synthetic cannabis until Friday is wrong.
''It's an unusual signal to send to the community; one, that this drug is harmful, but two, that you've got four days to respond to this particluar measure,'' he said.
''What the Government seems to be saying on the one hand is this is a dangerous drug we're going to outlaw it; on the other hand you've got four days to do what you like with it.''