Sydney’s Northern Beaches will enter lockdown from Saturday evening, after NSW Premier Gladys Berejiklian announced another 13 cases from testing on Friday.

From 5pm on Saturday until midnight on Wednesday, the local government area will revert to lockdown orders issued across the state in March.

People will only be permitted to leave their homes for five basic reasons: to seek medical care, exercise, grocery shop, work or for compassionate care reasons.

NSW has recorded 23 fresh COVID-19 cases in the past 24 hours, bringing the outbreak to a total of 40 cases.

“It seems apparent to us we will have the same number, if not more cases, tomorrow,’ Ms Berejiklian said.

Tweet from @NSWHealth

Twenty-one of those cases were locally acquired and linked to the Avalon cluster, and two remain under investigation.

Ms Berejiklian says the restrictions are essential if Sydney has any hope of a semi-normal Christmas.

“We’re hoping that will give us sufficient time to get on top of the virus, so that we can then ease up for Christmas and the New Year,” she said.

She also urged northern beaches residents not to panic buy.

“It’s okay to go out and do essential shopping so don’t panic buy, please.”

Ms Berejiklian has also asked the rest of Sydney to avoid unnecessary activity, and flagged the possibility of further restrictions being put in place tomorrow for the Greater Sydney area.

She again urged her interstate counterparts to take a proportional response to the outbreak.

“I would have done the same had I been in their shoes in terms of declaring the northern beaches a hotspot,” she said.

“It is a hotspot, no denying that, but… there’s no evidence that it’s outside northern beaches.”

Acting Chief Medical Officer Paul Kelly said the community had begun self-isolating and businesses and places of worship had closed their doors even before Premier Gladys Berejiklian announced the strict lockdown.

Key shopping precincts in the peninsula community are quiet and residents have been seen lining up to get tested.

“It’s extraordinary, it’s a wonder to watch,” Prof Kelly told reporters in Canberra on Saturday.

The official lockdown from Saturday to Wednesday would simply reinforce what was already happening, he said.

Meanwhile, hundreds of gym goers on Sydney’s Northern Beaches have been asked to get tested and isolate immediately, as NSW Health races to contain the clusters.

Authorities issued a public health alert on Saturday morning, calling several hundred people who attended an Avalon gym to take urgent action.

A known case attended Anytime Fitness on Avalon Parade on December 6, 7, 8, 11 and 12 while infectious.

Investigations are underway and authorities are in the process of tracking down close contacts but are asking anyone who attended the gym on those days to get tested immediately and isolate until further advice.

The gym adds to a lengthy list of venues visited by confirmed cases published by authorities on Friday afternoon.

They include numerous places around the northern beaches but also Cronulla RSL Club, Westfield Bondi Junction, a Turramurra hair salon, a Woolloomooloo restaurant and other Sydney venues and transport routes.

The warning periods range from December 11 to December 16.

Tweet from @NSWHealth

On Friday afternoon, NSW Health issued a 72-hour advisory to wear masks when indoors. It didn’t extend beyond the beaches region.

The state Labor opposition says the measure doesn’t go far enough, calling on the government to make masks mandatory on public transport, in supermarkets and shopping centres and in places of worship.

Other states acted on Thursday and Friday to prevent the cluster jumping borders, with more barriers for NSW residents erected by Western Australia, Victoria, Queensland, Tasmania and the Northern Territory.

Anyone who enters WA from NSW will be forced into 14 days of self-isolation and people who have been in the northern beaches will not be able to enter Victoria or Queensland without quarantining.

Genomic sequencing has connected the cluster to a US strain of the virus, which may have entered NSW in a returned traveller in early December.

That traveller has never left the hotel quarantine system, meaning the connection between that case and the northern beaches remains unclear.

“If we get on top of this in the next two or three days, all of us will be able to have a much better Christmas,” Premier Gladys Berejiklian told reporters on Friday.

Ms Berejiklian admitted recommendations may tighten if COVID-19 case numbers on the northern beaches don’t flatten. That included the enforcement of mask usage.

With many Australians planning holiday travel in coming days and weeks, all eyes will be on the Sydney cluster.

-with AAP