Another 15 COVID cases were recorded in NSW overnight, with hopes the state’s contact tracing team may have gotten this concerning cluster under control.

Premier Gladys Berejiklian also called on premiers around the nation to be “compassionate” and consider “heartbreak” associated with border closures, as state and territory leaders slam their doors shut to Greater Sydney.

More than 38,500 tests were carried out statewide on Sunday, the Premier announced on Monday, which she said was a record number. In the face of the heartening results – new cases just half of what was recorded on Sunday – the state government is still resisting making masks mandatory.

“Our continued hard effort is required to make sure that we continue that downward trend. But as we know in a pandemic, things can be volatile and can move quickly,” Ms Berejiklian said.

She praised NSW Health’s team as perhaps “the best contact tracing team on the planet”.

NSW recorded 15 locally acquired cases of COVID-19 in the 24 hours to 8pm last night, and an additional eight cases in returned travellers in hotel quarantine. pic.twitter.com/BBkUgXZELD

— NSW Health (@NSWHealth) December 21, 2020

But state health authorities are still scrambling to clamp down on the concerning cluster. NSW Health has not found the original source for the outbreak, with other state premiers now requiring people coming from Greater Sydney and surrounding areas to quarantine on arrival.

It comes after NSW logged 30 cases on Sunday, with new rules restricting gathering sizes and hospitality venue capacities.

Ms Berejiklian asked for her fellow state leaders to “think about the facts” when shutting their borders so close to Christmas, urging that a response “has to be proportionate”. She noted NSW had only ever shut its border to Victoria alone, and only when case numbers hit 140.

“All I’m saying to colleagues around the country is, please think about the heartbreak and please think about the facts when you’re making these decisions, because it impacts so many people,” she said.

“Yes, it’s volatile and we’re down to 15 cases and I hope that that trend continues.”

The current rules extend until at least Wednesday, the day before Christmas Eve. Ms Berejiklian said officials would make a decision about Christmas and New Year’s, and whether the rules would be extended beyond their current expiry, on Tuesday or Wednesday once those days’ case numbers were known.

Chief health officer Dr Kerry Chant said that decision would be based largely on whether there was any community transmission recorded outside the Northern Beaches epicentre. Almost all NSW cases in the last week have been linked directly to that outbreak and not further afield.

Authorities have pleaded with people in the Greater Sydney area  – which they define as stretching from Wollongong in the south to the Blue Mountains in the west and the Central Coast in the north – to wear masks while in indoor venues like shops. However, face masks won’t be made mandatory in public, like in Melbourne.

“We are considering at all times additional measures that need to be in place. The feedback I’ve had from the Northern Beaches community is that everyone is being very compliant with the requirements around wearing masks indoors,” Dr Chant said.

NSW recorded 15 locally acquired cases of COVID-19 in the 24 hours to 8pm last night, and an additional eight cases in returned travellers in hotel quarantine. pic.twitter.com/BBkUgXZELD

— NSW Health (@NSWHealth) December 21, 2020

There were a record 38,578 tests reported to 8pm last night, compared with 28,210 in the previous 24 hours. This has been the highest amount of COVID-19 testing ever done in a single day.

— NSW Health (@NSWHealth) December 21, 2020

The Premier praised health teams for doing a “phenomenal” job but called on businesses and patrons to take even greater care in their contact tracing and visitor logs.

“You shouldn’t be opening your doors as a business or an organisation unless you have your systems in place to get all of the information for people walking through that front door,” Ms Berejiklian said.

Health minister Brad Hazzard said some venue patrons were still writing false phone numbers or names when checking into venues, which made contact tracing even harder.

“Some of the visitors to various venues still think that it is funny to be caught putting in there that you’re Donald Duck or Mickey Mouse or a false phone number. That must stop,” he said.

“This is a worldwide COVID pandemic. And thinking it’s smart to call yourself Donald Duck or Mickey Mouse is about as stupid as it gets. So that must cease.”

He also noted that some people had written “illegible contact details” on paper forms. Mr Hazzard said it was mandatory for venues to have a QR code check-in service, but he did not plan to mandate venues to use the specialised NSW government QR code app, saying to do so would be “punitive”.

Dr Kerry Chant on Sunday. Photo: AAP

Dr Chant praised NSW residents for getting tested. She admitted there were “delays” in testing queues, with many people reporting having to wait for hours for a swab, but that the “majority” of results were still coming back within 24 hours.

The Premier and NSW Health also urged anyone in the Greater Sydney area to wear masks while in indoor venues.

Mr Hazzard and Dr Chant were both wearing masks during the press conference.