The Northern Territory has declared Sydney's northern beaches a coronavirus hotspot, forcing anyone from that area into 14-days of supervised quarantine in Darwin.
Acting Chief Minister Nicole Manison said travellers would be given the opportunity to return to NSW.
People in the NT who visited Sydney's northern beaches since December 11 are also being asked to self-isolate immediately and get tested and remain in isolation until their tests results are known.
Ms Manison said the hotspot area could be widened should the situation in NSW continue to deteriorate.
"This is a global pandemic and things can change quickly. We must always be ready to act," Ms Manison said.
"This is not the news we wanted to be delivering just one week out from Christmas.
"But throughout this crisis, we have warned everyone that things can change and we have to do whatever it takes to keep the Territory safe."
Chief Health Officer Hugh Heggie said the outbreak showed that the "war against COVID-19 is not over.".
But he said the Territory had prepared for such an eventuality and its decisions were "proportionate" to keep everyone in the NT safe.
