It comes as South Australia recorded two new COVID-19 cases today – a man and woman in their 20s who both recently returned from overseas and have been quarantining in a medi-hotel.

Meanwhile, the number of cases in New South Wales linked to the so-called Avalon cluster has spiked to 144, after 10 new cases of community transmission were announced this morning off the back of yesterday’s 18 reported cases.

The state’s second cluster – at Croydon in Sydney’s inner-west – grew by three to nine.

Victoria has reported six new cases in the past 48-hours – its first acquired infections in two months.

SA emergency coordinator and Police Commissioner Grant Stevens this morning announced the state would impose a hard border closure with New South Wales from New Year’s Day.

The restriction will ban people travelling from NSW into SA, unless they are an essential worker, are permanently relocating to SA or are a South Australian resident returning home.

Some exceptions will be granted for people on compassionate grounds, such as if they need to attend to a dying relative.

Those allowed into the state will be required to quarantine for 14 days upon arriving.

A 100-kilometre-radius “buffer zone” will be put in place for cross-border communities including Broken Hill and Wentworth, allowing residents of those towns to travel freely between the states.

People living in other states will be allowed to transit through NSW en route to SA, provided they do not stop in the eastern state for longer than necessary and can prove their travel is the most direct route.

SA has made no change to its current border arrangements with Victoria, but authorities are monitoring the situation in that state closely and say they are prepared to act if the number of cases linked to community transition increases.

Premier Steven Marshall described the recent outbreak of local coronavirus transmission in Australia as “pretty disturbing”, saying SA’s relatively COVID-free status was “really enviable” and the Government wanted to do everything it could to keep the spread of the virus into the state at bay.

“There’s no doubt that these are harsh restrictions and we take these decisions very, very carefully, but the reality is they are needed,” he said.

“We’re not going to keep them in place one day longer than we need to, but at this stage they need to be in place at this time.”

Under the current restrictions, anyone who arrives in SA from designated hotspot areas of NSW, including the Greater Sydney Region, Wollongong or Central Coast local government areas after December 21 needs to quarantine for a fortnight and have COVID-19 tests on their first, fifth and twelfth day of isolation.

This situation is volatile and changing rapidly

Those who visited the Northern Beaches council area for a 10-day period from December 11 are banned from entering SA.

Marshall said he was unsure how many South Australians were still in NSW, but “any South Australian who has been in New South Wales has been very aware of the developing position with regards to the Northern Beaches cluster”.

“I don’t think it would be any surprise to them whatsoever, in fact, SAPOL last night put out a further message to New South Wales making it very clear that we would be making an announcement this morning,” he said.

SA Police’s message last night warned South Australians contemplating travel into NSW to “very seriously consider whether that travel is essential”.

“Further, those people who are holidaying or travelling in New South Wales should consider returning to South Australia as soon as possible,” it said.

“This situation is volatile and changing rapidly.”

Police are currently manning the SA-NSW border crossing points to ensure all people entering the state have submitted a cross border travel application.

Stevens said there was no cut-off point for South Australians in NSW to return home, but those wishing to do so could only travel across the border once.

“Don’t think that you can manipulate the system by travelling backwards and forwards between New South Wales and South Australia claiming that you are a returning resident every time,” he said.

“It’s a one-time deal.”

South Australia’s hard border closure with Victoria earlier in the year banned everyone in that state – including South Australian residents – from travelling into SA.

Stevens said if the situation in NSW worsened, authorities would consider implementing a similar rule with their hard border arrangements with NSW.

“We’re hopeful that we don’t have to get to that point, but we have an obligation to make sure we take as many steps possible to limit the potential for COVID-19 to seep into the South Australian community and put us at a situation where we’re having to change our internal restrictions.”

It is still not clear how the Northern Beaches cluster started and health authorities are concerned about a growing number of positive cases emerging outside that hotspot area.

Stevens said authorities were not as concerned about Victoria’s current virus spread, as all positive cases in that state were linked to the NSW cluster.

“On the basis of their contact tracing efforts and putting people into quarantine who have been in close contact, we’ll just wait and see what happens.”

Meanwhile, more police officers than ever before will be on call tonight to monitor New Year’s Eve celebrations, to ensure compliance with COVID-19 directions and to monitor border check-points.

No more than 50 people are allowed to gather at people’s houses, while licensed venues must comply with a one-person per two-square-metre density requirement.

Stevens said he could not say when the private gathering cap would be lifted, but “one factor that would be taken into account is exactly what’s happening in other states and territories”.

“If we see that there’s a particular concern about the incidents of COVID-19 in another state it may be a factor that we take into consideration when freeing up even more activity within South Australia.

“The caps that we currently have in place will still stay into the New Year until the transition committee can meet and discuss what the alternatives might be.”

Assistant Commissioner Ian Parrott told reporters yesterday that police would tonight be out in force across metropolitan and regional South Australia, patrolling streets and responding to tip-offs about people breaching restrictions.

“As usual, we will be focussing on public safety and public behaviour, particularly in places where people would normally gather at this time of year, but also in the absence of major events and major fireworks, we still anticipate people to be going to coastal areas and sometimes to the CBD,” he said.

“We’re also aware that people will likely take a more cautious approach and hold gatherings at households and homes, and celebrate in a more subdued or quiet way.

“We will be having similar numbers of police on duty right throughout New Year’s Eve, but we will be more mobile – we will be out in the suburbs, we will be responding to calls for assistance and we will also be enforcing COVID-compliance.”

South Australia currently has seven active cases of COVID-19 and a reported total of 576 cases since the start of the pandemic.

Chief public health officer Professor Nicola Spurrier said the imposition of the hard border closure with New South Wales was a reminder not to be complacent about the spread of COVID-19.

“All of us as citizens have an obligation to continue that basic hygiene – basic hand-washing, hand sanitising, one and a half metres staying away from people that you don’t know,” she said.

“It’s fun to go out, it’s fun to join in the celebrations, but you really need to take heed of the basic hygiene messaging.”