Australia banned flights from nine southern African countries on Saturday, tightening its borders to prevent the entry of the new Covid-19 Omicron variant.

Non-Australians who visited South Africa, Zimbabwe and several other nations in the past fortnight will also be barred from Australia, Heath Minister Greg Hunt said.

Citizens and residents travelling from the listed countries will have to quarantine for 14 days.

"These are strong, swift, decisive and immediate actions," Hunt told media in Canberra.

The variant -- which has a large number of mutations -- was first detected on November 9.

Scientists are racing to understand how it behaves, but there are fears the strain may be more transmissible or render existing vaccines less effective.

The World Health Organisation listed Omicron as a variant of concern and said it could take several weeks to know if there are significant changes in transmissibility, severity or implications for Covid vaccines, tests and treatments.