A territory-wide lockout of unvaccinated people started at 1pm local time on Thursday as community cases surged.
"Today's escalation in case numbers is concerning," Chief Minister Michael Gunner told reporters.
"Our community transmission rate has grown in recent days".
All told, 27 new community transmission cases were detected, up from three two days ago but down from 34 on Wednesday.
Mr Gunner said the increase was a result of New Year's Eve social gatherings with more than 50 per cent of new infections diagnosed in people aged 20 to 39.
Ten of the new cases are known close contacts, 112 remain under investigation and 107 are interstate and international arrivals.
Mr Gunner said the territory's virus reproduction rate had climbed to 4.5 and about 10 per cent of tests were yielding positive results.
He announced a territory-wide lockout from 1pm on Thursday until noon on Monday when a vaccine-pass system will be introduced.
Of the 256 cases, two were recorded in Tennant Creek, 510km north of Alice Springs, as the Delta outbreak that started in Darwin in late October continues to grow.
Eighteen people tested positive in in Yulara, 18km from Uluru, as the virus spreads through the community of about 1000 people.
Twenty-three COVID-19-infected people are in NT hospitals, with eight cases suffering severe symptoms in Royal Darwin Hospital.
Two of those are in intensive care, with one on a ventilator.
The territory recorded 117 new cases on Wednesday and 75 on Tuesday.
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