SYDNEY - Australia's domestic borders have been slammed shut on residents in Sydney, as a COVID-19 outbreak in the city's north increased by 15 cases to 83 on Monday.

Concerns over how far the cluster has spread prompted differing responses by state leaders across the country.

The state of Western Australia re-implemented a hard border closure for the entire State of New South Wales (NSW) of which Sydney is the capital, turning back anyone from NSW without a legitimate exemption.

Victoria, Queensland closed their borders just to the Greater Sydney region, allowing residents of those states until the end of Monday to get back across the border before restrictions apply.

Meanwhile South Australia and Tasmania introduced softer restrictions requiring arrivals from the Greater Sydney region to quarantine for 14 days.

The area where the outbreak originated, Sydney's Northern Beaches, remained in lockdown until Wednesday while residents from the Greater Sydney area were asked to limit their movements.

Conditions on residents in the Northern Beaches were similar to those during NSW's initial outbreak of COVID-19 back in March, allowing them to leave their homes only to work, shop for necessary items, provide care or take brief isolated exercise.

On Sunday, NSW recorded its highest number of tests conducted in a single day, with 38,578 people tested.

NSW Premier Gladys Berejiklian told reporters that a tightening of restrictions over Christmas would take place if warranted by the numbers of new cases over the coming week.