SUNRAYSIA businesses and sporting competitions were dealt another hammer blow as Daniel Andrews thrust Victoria into lockdown late on Thursday.

Despite demanding NSW Premier Gladys Berejiklian put a ring of steel around Sydney, the Victorian Premier again refused to do the same to Melbourne to contain a growing outbreak of the COVID-19 Delta variant in the western suburbs.

The state's sixth lockdown was prompted in part by fears a teacher at Al-Taqwa College in Truganina, infected with COVID-19, may have unknowingly spread the virus.

Mr Andrews gave Victorians just four hours' notice of the lockdown, which started at 8pm.

The same rules that applied during last month's lockdown were reimposed, including the 5km travel limit for exercise and shopping and compulsory masks indoors and outdoors.

"There is no alternative for us but to listen to our public health experts, take their advice, and make the decision – the very difficult decision – my Cabinet colleagues and I have made on the advice, and that is Victoria will have a lockdown again for seven days," Mr Andrews said.

Victorian Nationals leader Peter Walsh said the statewide lockdowns were killing country communities.

"Just a week ago, the Premier was arrogantly verballing New South Wales Premier Gladys Berejiklian over the Sydney outbreak, saying, 'A ring of steel will work, it will, and that's why I called for it'," Mr Walsh said.

"Yet Daniel Andrews pig-headedly refuses to take his own advice and follow through with a proportionate, localised response here in Victoria.

"While Daniel Andrews and his ministers haven't missed a single pay cheque in the past 18 months, there's tens of thousands of Victorians who are out of work as a result of Labor's lockdowns."

The state's fifth lockdown ended just nine days ago.

Mr Andrews said even though all the cases announced on Thursday were in Melbourne, sewage tests had detected virus fragments in the Wangaratta region.

"As painful as it is, and counterintuitive sometimes as it is, we don't want this taking hold in country Victoria," he said.

Mr Andrews also made mention of the recent case in Mildura, where a man in his 30s contracted the virus while at a football match at the MCG. The virus was contained to one household and they remained the only cases in Mildura for almost 18 months.

More than 16 million Australians are in lockdown across the country.