During his COVID-19 update on August 19, Mr Gunner said the reason for the lockdown extension, that was initially planned for 72 hours, is largely due to how infectious the index case was whilst in the area.

"I'm sorry you have to do this for another 24 hours. We need Katherine to hang tight for another day," Mr Gunner said.

"Given that we know the man was more infectious while in the community, given the high traffic of the exposure site in Katherine, given the movement between Katherine and remote communities, and given the number of tests that we are doing there..."

In a first for Australia, the Northern Territory has declared the Sydney and Canberra airports as hotspots for the purposes of travel to the Top End from 5pm on August 19, 2021.

"Usually airports are exempt from hotspot declarations but because of this concerning case, we need to assume these airports are too risky for transport until we know more," Mr Gunner said.

"This is about plugging any holes in our armour."

Changes to the day three testing have been made which will require people who arrive in the NT following hotel quarantine to be in supervised quarantine from their arrival until a negative result has been returned.

"We added the three day rule as an extra layer of protection, now we are strengthening that rule to strengthen our defence against COVID-19," Mr Gunner said.

Mr Gunner confirmed the index case has been infected with the Delta variant, and described the source of infection as a "mystery".

"Right now, the source of the infection remains a mystery. It's probably one of the most unusual cases we have seen in Australia," he said.

"It will take a few more weeks and a lot more work to solve this mystery."

Waste water results for the Katherine area have returned a weak positive with testing of fragments to continue to ensure detection disappears.

Approximately 2,403 COVID tests were conducted yesterday with all returning a negative result, including 87 of whom were close contacts.

Of the 626 casual contacts, 272 are negative.

"I thank all close and casual contacts for their help this week, in particular by the people of Katherine where most of our casual contact has occurred," Mr Gunner said.

Residents are reminded to continue to use the QR code check-in as well as the use of masks which will remain for one week after the lockdown has been lifted.

"Masks must be worn when you are in public and you must maintain a 1.5 metre distance. If you travel to another part of the territory, you take these rules with you when you travel," Mr Gunner said.

"Our QR code checking in system has never been more important."

The Katherine lockdown is planned to end at noon on August 20, 2021.

More to come.

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