Darwin and surrounding areas will enter lockdown for 48 hours following four new COVID-19 cases linked to a central Australian mine.
None were locally transmitted but Chief Minister Michael Gunner says the outbreak represents the Northern Territory's biggest crisis since the beginning of the pandemic.
He says the cases involve the highly contagious Delta variant and more infections are expected.
The lockdown, which begins at 1pm on Sunday, follows revelations that 900 workers who left Granites Mine 540km northwest of Alice Springs where a man tested positive, flew to Brisbane, Perth and Alice Springs.
Of 244 potentially exposed people who remained in the NT, Mr Gunner says 15 who arrived in Darwin since Friday remain unaccounted for.
As a result, there was a need to "assume the worst, assume they are positive and assume that there are exposure sites".
Mr Gunner says one of the four detected positive cases had travelled to NSW and was being managed by authorities there.
Two from the cohort were still isolating at the mine in the Tanami Desert but would be evacuated to the Territory's Centre for National Resilience at Howard Springs.
The fourth case was one of the mine worker's close contacts who lived in Palmerston, south of Darwin, and had tested positive to COVID-19 while in the Centre for National Resilience.
Mr Gunner stressed that the cases were not considered community transmissions.
He said the Palmerston case involved a 64-year-old employee of the mine who had travel by plane to Darwin on Friday.
He was picked up by his wife and went straight home to Palmerston.
His only other travel was to collect his adult daughter from her workplace and he did not leave the car.
His wife and daughter had no additional movements.
When interviewed on Saturday, the man told contact tracers he was isolating at home with his wife and daughter and had developed symptoms.
From there, he was immediately tested and taken to the Centre for National Resilience.
Mr Gunner said two contacts of the original mine case were still to be located.
"We are urgently tracking them down but again ... we are assuming the worst.
"Everything we see points to this being the highly infectious Delta variant," he added.
"We are expecting more cases and we are not expecting them to be as clear as ... this morning.
"There is a stronger chance that any new cases will have exposure sites which makes the job of tracing and testing much bigger."
Darwin, Palmerston and Lichfield local government areas will enter full lockdown for 48 hours.
Residents within are only permitted to leave home for medical treatment, to obtain essential goods and services, for work considered essential, one hour of exercise a day or to provide care.
Anyone who leaves home must wear a mask.
© AAP 2021