The Northern Territory’s lockdown in greater Darwin has ended as planned after no new COVID-19 cases linked to a US defence contractor were found.

However, the town of Katherine will remain in lockdown for a further 24 hours as authorities wait for test results from people who could have had contact with the infected man.

About 150,000 people were locked down for three days on Monday after the man in his 30s travelled from Sydney via Canberra to Darwin.

The greater Darwin lockdown ended as planned at midday on Thursday, but Katherine, 320 kilometres south, will remain under restrictions until Friday.

“We need Katherine to hang tight for another day,” Chief Minister Michael Gunner said on Thursday.

Mr Gunner said the man was more infectious while he was in Katherine and the town of 8000 is a busy hub with links to many remote Aboriginal communities.

Contact tracers are also yet to find 12 people who potentially crossed paths with the man at the local Woolworths.

“I am sorry you have to do this for another 24 hours. This is the recommendation of our health experts,” Mr Gunner said.

Authorities still don’t know how and where the man contracted the virus.

The infection has been confirmed as a strain of the Delta variant, but it hasn’t yet been linked to any other cases in the country.

“The source of this infection remains a mystery,” Mr Gunner said.

“It is probably one of the most unusual cases we have seen in Australia.”

As a result, the NT will declare Sydney and Canberra airports hotspots from 5pm on Thursday, with concerns the man may have contracted the virus at one of them.

The airports, which are usually exempt, join all of NSW, the ACT and greater Melbourne on the NT hotspot list.

Only essential travellers are allowed into the NT from those locations and they must complete 14 days in quarantine.

“Because of this concerning case, we need to assume that these two airports are too risky for transit until we know more about them,” Mr Gunner said.

The Territory’s mandatory COVID-19 test for travellers who have completed hotel quarantine in other jurisdictions on the third day after they arrive in the NT has also changed.

They’ll now be required to stay in supervised quarantine from the time they arrive in the NT until they return a negative test.

Some health restrictions will remain in place in the lockdown areas for seven days.

These include wearing face masks in public when people can’t socially distance themselves.

Patrons will need to be seated at hospitality venues. Gyms and markets will remain closed for a further week.

People can have a maximum of 10 visitors to their homes

Schools will reopen on Friday and Darwin Festival events can resume.

The infected man in his 30s arrived at Darwin Airport on a Qantas flight just before midnight last Thursday and travelled to the Hilton Hotel by taxi.

He returned a positive result on Sunday night after mandatory testing at Royal Darwin Hospital earlier in the day.

Before the results arrived, he drove to Katherine for work, where he met a friend at a resort and visited Woolworths.

He previously returned a negative COVID-19 test on August 10 during his 14-day stay in a Sydney quarantine hotel.

-AAP