SYDNEY, Jan. 4 (Xinhua) -- Australia's biggest city of Sydney officially recorded zero locally acquired cases of COVID-19 on Monday, for the first time since Dec. 15, 2020.

Despite the promising figure, health officials remained on edge over a cluster developing in Sydney's western suburbs.

Officials noted that overnight two cases were linked to the cluster at a liquor store in the suburb of Berala, although due to being recorded outside the official reporting period on Monday, the cases will be included in Tuesday's figures.

Anyone who attended the liquor store or a nearby supermarket during several periods between Dec. 22 and Dec. 31 were considered close contacts and told to get tested and isolate for 14 days, regardless if they received a negative result.

"That is a long time since that period, but what we want to do is pick up any people that may have been transmitted to then, to assist us in blocking any further chains of transmission," New South Wales Chief Health Officer Dr Kerry Chant said.

Being the peak festive season in Australia, thousands of people were expected to be required to self isolate, in addition to thousands more already considered close contacts since the most recent outbreak began.

Health officials said that testing rates had dropped to an insufficient level below 30,000 per day and urged anyone with even mild symptoms to get tested.

"We are particularly urging residents of western, southwestern Sydney and Wollongong to come forward with even the mildest of symptoms," Chant said.

"We need to see those testing levels sustained and that's because people were at those exposure venues for a number of days."

In addition, starting Monday mandatory face masks were introduced for Sydneysiders in public indoor spaces and on public transport for the first time since the pandemic began.

Meanwhile the state of Victoria recorded three locally acquired cases, linked to a cluster which was said to have originated in Sydney.

Acting Victorian Premier Jacinta Allan said that the relatively small figure which came on the back of high testing numbers was good news for the city.

"There is great work being done by our tracing teams, and the testing regime is out in force," Allan said.

"Victorians are doing a great job following the rules and practising COVID-safe behavior which is contributing to a low number of cases."