The Northern Territory has recorded no new cases of COVID-19 overnight.
Chief Minister Michael Gunner said 1,846 COVID-19 tests were carried out on Monday, all of which returned negative results.
"We're not out of the woods yet, but so far, so good," Mr Gunner said.
Today is the first full day of a snap 72-hour lockdown across the Greater Darwin and Katherine regions.
On Monday a man in his 30s who travelled from the US and spent 14 days in hotel quarantine in Sydney before arriving in Darwin tested positive.
The man tested negative before leaving quarantine, but was retested in the NT, as required, and returned a positive result overnight on Sunday.
He is believed to have been infectious for several days while travelling from Darwin to Katherine for work.
Mr Gunner said the man had since been tested again, and returned another positive result.
"His second test was more infectious than his first, that means that we detected the infection early," he said.
"It also indicates that he was more infectious in Katherine then he was in Darwin, which is why the Katherine testing is so important today."
99 close contacts identified
Mr Gunner said a friend who spent time with the man in Katherine had tested negative, as did a taxi driver and an Uber driver who drove the man between the airport and his hotel in Darwin on Thursday night.
Both drivers have been transferred to Howard Springs quarantine facility for 14 days, where they will remain as close contacts.
Ten people used the same Uber as the positive man and have also been identified as close contacts, and health authorities are in the process of tracking them down.
More than 300 people have been identified as close and casual contacts, including 32 people who were on the man’s flight from Sydney to Darwin via Canberra.
Eleven people from the Hilton Hotel in Darwin’s CBD, where the man stayed, have also been identified as close contacts.
Altogether, Mr Gunner said 99 people had been identified as close contacts.
"Thirty-two are in Howard Springs or are being transferred there, 41 are in other appropriate forms of isolation, 11 have travelled interstate and those authorities have been identified," he said.
"Fifteen close contacts remain unaccounted for, and 10 of those are from Uber that we’ve just identified."
Mr Gunner said the NT was now playing a "waiting game" and authorities would learn more as more test results came back.
"We believe we have done everything we can to trap the virus, we just need to wait and see what the outstanding tests come back as," he said.
"The wastewater from Darwin is being tested today and we will get those results late tomorrow."
Mr Gunner said Katherine wastewater was also being tested, with results expected back before Thursday night.
He urged people to keep checking in when they visited venues across the Northern Territory.
Variant, source of infection still unknown
Mr Gunner said health authorities were yet to determine the source of the man’s infection.
"Right now, while we’re in the middle of a lockdown, I’m not going to speculate where it came from or what it means for our border protection or quarantine policy in the future," he said.
NT Chief Health Officer Hugh Heggie said investigations were ongoing.
"It may have been on an airplane or at an airport, that's yet to be determined," he said.
Dr Heggie said genomic testing results were expected to be returned later today, but authorities assumed the man had the Delta variant.
The NT's 72-hour lockdown is scheduled to lift at midday Thursday.
"It may need to be extended if we don't have all the information required," Dr Heggie said.
He said he "strongly encouraged" people living in remote communities to wear masks and to avoid gathering in large groups, as a precautionary measure.
