19 Authorities are on high alert after new cluster of infections was detected in New South Wales.

28 cases have been linked to Sydney's northern coastal suburbs, but health officials have been unable to determine the source of the infections, raising fears that the cluster could spread.

State Premier Gladys periodically in has urged people to be diligent, so everybody in greater Sydney needs to be on high alert.

If you have the modest of symptoms, please get tested and isolate.

But also can everybody please think about their activity?

Nobody should be getting on public transport without wearing a mask.

Nobody in greater Sydney should be going to a supermarket, a place of worship or other high risk areas without wearing a mask.

Australian states and territories started reimposing border restrictions on Friday, causing disruption to thousands of People's Christmas travel plans.

It's also a blow for the economy and domestic tourism operators.