There are at least four new cases linked to a Northern Territory miner who tested positive for Covid-19 on Saturday.
NT Chief Minister Michael Gunner said the territory is facing its biggest threat "since this crisis began" and believes the Delta variant has escaped into the community.
Greater Darwin, Palmerston and Litchfield will go into a snap 48-hour lockdown coming into effect at 1pm on Sunday.
"We are taking extreme action right now to stop or slow any spread before the coronavirus," he said.
"It has been let loose in the territory and that means we need a lockdown."
Mr Gunner said he could not rule out the lockdown going for longer.
"We know how infectious this can be. One case turns to two, two into four, we know what happens," he told reporters on Sunday.
"I would rather regret us going too hard, too early than going too easy and risk it all."
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