NT Chief Minister Michael Gunner said a mysterious Covid-19 case in Darwin is the "most unusual case" seen in Australia, with authorities still unable to link it to any other case in the country.
The NT announced a 72-hour snap lockdown on Monday after a man in his 30s travelled from the US to Sydney via Canberra, before travelling to Darwin.
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.
"We cannot yet link the particular specific strain to any other case anywhere else in Australia," Mr Gunner said on Thursday.
"It'll take a few more weeks and a lot more work from experts around the country to solve this mystery."
The man flew from the US to the NT via Sydney, where he completed his two weeks in hotel quarantine.
He then travelled to Darwin via Canberra last Thursday, returning a positive result on Sunday night after being in Darwin for three days.
Before he received the results, he drove to Katherine for work, where he met a friend at a resort and also visited Woolworths.
The man had returned a negative Covid-19 test on August 10 during his hotel quarantine stay.
Greater Darwin out of lockdown
Greater Darwin's lockdown ended as planned at midday on Thursday after no new Covid cases linked to the man were found.
The town of Katherine will remain in lockdown for a further 24 hours.
Sydney and Canberra airports were declared as hotspots for the territory due to concern over cases picked up in transit.
"This is to plug any holes in our armour," Mr Gunner explained.
He also announced a change to the day-three testing rule, saying anyone arriving in the NT from hotel quarantine must now be in supervised quarantine from the time of their arrival to receiving their results.
"We added the day three rule as an extra layer of protection for the territory," Mr Gunner said. "Now we are strengthening our defence against Covid-19."
As of Thursday, Sydney, Melbourne and Canberra remain in lockdown.
The man was 'very early' in his infection while in the NT
The NT chief health officer, Dr Hugh Heggie, said early Covid tests showed the man was "very early" in his infection while in the NT.
Dr Heggie said it was likely the man became infected between leaving hotel quarantine in Sydney and arriving in Darwin, but it's unclear how or where exactly he was infected.
Dr Heggie said genomic tests had since confirmed he had the Delta strain of the virus.
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