New South Wales Premier Gladys Berejiklian has reported 23 new cases of coronavirus in the state and will impose a lockdown for the Northern Beaches area from 5pm (local time) today.
Twenty-one of the new cases are linked to the Northern beaches cluster and two remain under investigation.
The Premier said it is likely the state will see a repeat of those numbers in the days to come and a localised, mandatory lockdown was necessary.
"Based on the likelihood that we are going to expect to see a repeat of those numbers across this 24 hours, given what we know from the testing, given what we know from the likely number of cases, we are going to put in health orders for the northern beaches from 5 o'clock today to essentially go back to the restrictions people experienced in March," she said.
That means, for residents within the Northern Beaches area, only leaving home for essential purposes such as for work, for compassionate grounds, for exercise or to go shopping.
The stay-at-home order will go into effect from today and continue through to midnight on Wednesday.
Berejiklian also urged people who might have plans around the city to "consider changing those plans on staying at home", warning "we do not want the virus to spread outside of the Northern Beaches".
NSW Health Minister Brad Hazzard told northern beaches locals to cancel any plans they have and to "take it seriously" as the outbreak continues to grow.
"If you have got lunches with a lot of friends today, cancel it. If you are going out for big functions this afternoon, cancel it," Hazzard said.
"If you are going to a gym, hopefully the gyms have closed, they will be closed by 5pm but I would not be going to a gym. We have an outbreak in Avalon, we do know that the numbers are increasing but we do know that we need to be really cautious."
Mystery surrounds 'patient zero'
NSW health authorities are still not yet confident they have found the original source of the outbreak, making it difficult to ensure every transmission route is being investigated.
Genomic testing shows the virus likely came from the US, however NSW chief health officer Kerry Chant said on Thursday her "anxiety" was that "we haven't found the direct transmission route".
This means officials cannot be sure that they've "locked every transmission line".
University of Queensland virologist Professor Ian Mackay told the ABC finding the original source was critical.
"If we can find the index case, that means we can work forward from there to make sure all the contacts have been identified," he said.
"If we can't find the source, there's always a possibility that some cases have been missed."
Northern Beaches in 'voluntary' lockdown
Photos have shown the Northern Beaches community already went into a "voluntary" lockdown last night.
The usually packed Manly Corso area was deserted on Friday night after 28 cases of coronavirus were linked to the beachside suburbs.
Across the Northern Beaches, the majority of pubs also opted to close.
Earlier on Friday, Berejiklian asked residents to stay in their homes for all but essential reasons. "Do not leave your home", Berejiklian implored the 250,000 people living in the Northern Beaches area.
Response from other states
Sydney's outbreak has led to a rapid response from Australia's other states and territories. An overview of each:
Victoria has introduced a traffic light system.
The northern beaches and other exposure sites are "red zones", and travellers are not permitted into Victoria from today without 14 days in quarantine.
Greater Sydney is an "orange zone", and people travelling to Victoria are encouraged to seek a test on arrival and self-quarantine until they receive a negative result.
Regional NSW is a "green zone", and people travelling to Victoria from this area are asked to monitor for symptoms.
Queensland has declared the Northern Beaches a coronavirus hotspot. Anyone who has been in a hot spot is not permitted to enter Queensland without 14 days of mandatory quarantine.
Western Australia has implemented mandatory 14-day quarantine for anyone arriving from NSW. The state remains open to introducing a hard border in the "near future".
In South Australia, anyone who has visited the northern beaches since December 11 will need to quarantine for 14 days.
The Northern Territory declared the northern beaches a coronavirus hotspot and will require residents from the area enter mandatory supervised quarantine.
In Tasmania, travellers who have been in the northern beaches need to apply for a travel permit. Tasmanians wanting to return to the state need to provide proof of residency and can complete their quarantine at home.