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.
Sydney records 30 new cases
NSW has recorded 30 new locally acquired cases of coronavirus on the first day of a 14-day lockdown for large parts of the state.
Premier Gladys Berejiklian has confirmed all 30 cases have been linked to the Bondi outbreak, while 11 of them were self-isolating throughout their infectious period.
A further three cases were in isolation for part of their infectious period.
The new cases bring the outbreak to 110, while a further two local cases remain under investigation.
More than 52,000 people were tested and 12,881 received a vaccine dose in the 24 hours to 8pm on Saturday.
NSW Chief Health Officer Kerry Chant said 10 of the new cases are linked to Great Ocean Foods in Marrickville, bringing that cluster to 11.
One case linked to the seafood distributor is a flight crew attendant with Virgin Australia who tested positive on Saturday night.
Anyone who received a delivery from the business between June 21 and June 25, plus their household members, are being urged to contact NSW Health, immediately isolate and get tested.
Other cases of concern to authorities are a person who tested positive after attending the COVID-19 vaccination centre at Westmead Hospital on June 22, and a close contact of a worker at the Granites gold mine in central Australia, who tested positive in the Hunter New England region of NSW.
"I can assure the community that this case was not infectious in the community and did not present any risk to the community of NSW," Dr Chant said.
She said the case is believed to have contracted the virus in Queensland.
Despite the jump in new cases, Dr Chant is reassured they are all linked.
She said the lockdown of Greater Sydney, the Blue Mountains, Central Coast, Wollongong and Shellharbour, which is scheduled to end at 11.59pm on July 9, would be sufficient to bring the outbreak out of control.
"Obviously, we have to be very cautious and look at the data every day but if we all take this very seriously, we maintain those testing numbers, then two weeks may be sufficient to have that comfort," Dr Chant said.
Ms Berejiklian said she expected case numbers to increase in the coming days, given the infectiousness of the Delta variant.
"Case numbers are likely to increase even beyond what we have seen today because we are seeing that people in isolation, unfortunately, would have already transmitted to all their house contacts," she told reporters on Sunday morning.
"The measure of our success won't be so much the people in isolation get the disease but the measure of our success will be to limit the number of people who went out and about into the community with the disease."
NSW residents in the lockdown zone are only allowed to leave home for work that can't be done at home, to shop for essential items, for exercise, to seek medical care or for caregiving or compassionate reasons.
Anyone in NSW who has been to Greater Sydney since June 21 is also being asked to stay at home for the lockdown period.
