THE Australian state of New South Wales has closed its southern border with hard-hit Victoria in a bid to limit the spread of coronavirus.

New South Wales (NSW) premier Gladys Berejiklian has long been a critic of states that close their borders to her state when its capital Sydney had Australia's largest numbers of Covid-19 cases.

But she had changed her stance on keeping Australia's internal borders open because the situation in the Victorian capital Melbourne was unprecedented.

The overwhelming majority of news cases detected in Melbourne in recent weeks were from community transmission.

Berejiklian said everywhere else in Australia, the vast majority of cases were infected overseas or had been infected by a returned traveler.

She said: "What is occurring in Victoria has not yet occurred anywhere else in Australia. It's a new part of the pandemic and, as such, it requires a new type of response."

NSW police will close the Victorian border later on Tuesday, she said, adding that some flights and trains services would continue for travellers who are given permits and exemptions.