Australia's drug regulator has warned consumers against buying a diet pill marketed as a natural product that actually contains a banned prescription substance.

Meizi Evolution Botanical Slimming soft gel capsules are being offered online but have not been assessed as safe by Australia's Therapeutic Goods Administration (TGA).

The TGA has tested the pills and found they contain an undeclared prescription substance, sibutramine, though the product label claims they contain only herbal ingredients.

The prescription-only medicine sibutramine was the active ingredient in weight-loss medicine Reductil, which was withdrawn in October 2010 after a study showed an increased risk of major cardiac events.

The TGA said the supply of the capsules was illegal and the place of manufacture had not been approved.

A number of people in Australia have bought the product online, the TGA said in a statement on Monday.

It advised people who have bought the pills to stop taking them immediately and take any remaining pills to a local pharmacy for safe disposal.

The TGA is working with Australian customs to stop future shipments of the pills from entering Australia.