Victoria will ban anyone who visits Sydney "red zones" from entering the state and require other NSW travellers to apply for a permit.

NSW travellers will need a permit to enter Victoria from midnight Friday, while anyone who has visited Sydney's northern beaches will be denied entry.

The city's northern peninsula virus cluster has grown to 28 cases and is expected to get bigger.

Victoria's new "traffic light" permit system means anyone who has been to the northern beaches or other "red zone" locations since December 11 will be shut out or forced to quarantine in a hotel on arrival.

People from the greater Sydney "orange zone" will be asked to take a test and self isolate, while regional NSW "green zone" residents will be asked to monitor for symptoms.

Anyone from NSW who is in Victoria and has even the slightest symptoms is being urged to get tested immediately.

Victoria's Health Minister Martin Foley says anyone travelling from Sydney should not visit Victoria.

"If you are coming from Sydney and you don't have to come, please don't come," he said.

At the same time, Mr Foley strongly advised Victorians not to visit Sydney, warning they may be forced to quarantine when they return.

"It won't be the Christmas or the holiday you were planning," he told reporters.

Permits will be available online and Victorian authorities will check them from midnight Friday in Melbourne, Albury, Mildura, Bendigo and Avalon airports, while police will patrol road borders.

Meanwhile health officials are contacting everyone who has flown from NSW to Victoria since December 11 and ordering those from "red zones" to get tested and self-quarantine for 14 days from the date they were last there.

Authorities are also conducting spot checks on 47 interstate flights scheduled to arrive in Melbourne on Friday.

Opposition Leader Michael O'Brien welcomed the system but said it could be better.

"There are some sensible, practical things the government could do to protect Victoria even further," he told reporters.

"That includes mandatory testing for people arriving from Sydney to Melbourne and ... setting up checkpoints for road entries rather than just random car lotto of vehicle patrols."

Victoria's Chief Health officer Brett Sutton says the NSW outbreak is concerning, with even more cases expected in coming days.

"We need to expect that it's going to be a number of weeks until this outbreak is under control," he said.

However the public should be reassured as most cases appear linked through visitors to the Avalon Bowling Club and RSL club.

Mr Sutton said one person in Victoria had identified themselves as a close contact of the northern beaches outbreak and is in hotel quarantine.

Victoria has gone 49 days without a locally-acquired case of COVID-19 but there have been eight cases in returned travellers confined to the state's new hotel quarantine program.