The situation in NSW is changing fast.

Case numbers are continuing to grow. There are still missing links in the chains of transmission, which suggests there'll be more cases out there that the authorities are yet to find.

Without mandatory masks and stay at home orders across Sydney, we do not have confidence that the situation remains safe.

That's why from 11:59pm tonight, the Greater Sydney area and the Central Coast will be designated a 'red zone'.

People who live in these areas or have visited since 11 December cannot enter our state.

Anyone from the red zone found trying to enter Victoria in breach of these orders will be subject to mandatory 14-day hotel quarantine.

For Victorians currently in Greater Sydney or on the Central Coast, you'll have until 11:59pm on Monday 21 December to return home.

You'll need to register for an exemption on the Service Victoria website and get tested within 24 hours of your return.

You'll then need to self-quarantine at home for 14 days from the date you were last in a red zone.

DHHS teams will be checking on people to make sure they have everything they need – and to make sure they're at home.