Ten new cases of COVID-19 have been reported in New South Wales, taking the Australian state's count to 28.
The cases are primarily located in Sydney's Northern Beaches, where a stay-at-home order is now in force. Some cases have been detected outside of the area, but are directly linked to that cluster. One such case was diagnosed in Queensland.
The exact origin of the cases remains unclear.
"The virus is likely of overseas origin. The source of infection is still being investigated," NSW Health says.
Premier Gladys Berejiklian warned people should stay away from high-risk areas. Masks should also be worn on public transport as well as to supermarkets and places of worship.
"We do have the potential to get on top of it, but it does require hard work on the behalf of all of us," the Premier said.
"Nobody should be getting on public transport without a mask. It would just be crazy if people are undertaking those activities without wearing a mask."
Those in Northern Beaches are being asked to work from home where possible, not visit friends or family in aged care facilities or hospitals unless essential, aovid unnecessary gatherings and high-risk venues such as clubs, restaurants, places or worships and gyms, and avoid unnecessary travel outside of or to the Northern Beaches area.
Authorities are concerned with the sharp rise in cases, especially due to the upcoming Christmas and summer break.
Six pop-up clinics were set up in the Northern Beaches on Thursday in response to new cases in the area, with thousands of people receiving tests.
"NSW Health has already seen an excellent response from the Northern Beaches community for testing, but it is absolutely vital everyone comes forward for testing isolates immediately if they have even the slightest of symptoms, such as headache, fatigue, fever, sore throat or runny nose."
Berejiklian earlier on Friday that NSW needed to brace for a "considerable amount" of COVID-19 cases.
Most other Australian states have imposed restrictions on travellers who have been in the Northern Beaches region. Western Australia will stop anyone from NSW entering the state unless they quarantine for two weeks.
Before the emergence of this cluster, Australia had largely contained community transmission of the coronavirus.
The new cases raise fresh questions over a potential trans-Tasman travel bubble, with the New Zealand Government saying one would only proceed should Australia go 28 days without a local case.