The Northern Territory has two new COVID-19 cases, including the first Top End health worker to be diagnosed with the virus.

It brings the territory’s latest outbreak to 21 after a 33-year-old non-Aboriginal man and a 59-year-old Aboriginal woman linked to the Katherine cluster became infected.

“The male is a health worker who initially had contact with an earlier positive case,” Chief Minister Michael Gunner said on Friday.

“The man was fully vaccinated but he also has other existing health issues so he has been transported to Royal Darwin Hospital.”

The woman has been linked to one household where other cases in the current cluster live. But she was not a known close or casual contact and discovered her COVID-positive status only after symptoms prompted her to get tested.

Meanwhile, genomic testing has confirmed the territory’s current outbreak was triggered by an infected woman who also illegally entered the NT.

The 21-year-old lied on her border entry form late last month before travelling from Cairns to Darwin after visiting Victoria, where she contracted the virus.

She infected a man she spent time with in Darwin and the virus spread to two others in Katherine before authorities declared they had control of the outbreak on November 9.

But that was short-lived, with the same strain of virus found to be responsible for the current cluster, which emerged in Katherine and the remote Indigenous community of Robinson River on Monday.

It started when a 30-year-old woman and a 43-year-old man from Katherine, 320 kilometres south of Darwin, became infected.

The woman was unvaccinated and travelled from Katherine to Robinson River – 1000 kilometres south-east of Darwin – where she tested positive. It was the first case reported in a remote NT Aboriginal community.

Nine new cases were detected in Katherine on Tuesday, including a 71-year-old man and a 65-year-old woman who was admitted to Royal Darwin Hospital.

Eight new cases were reported on Wednesday, with five infections diagnosed in Robinson River, including a three-week-old girl.

There were no cases on Thursday.

Health teams are concentrating on finding the missing links between the two clusters, with NT Health saying it knows the virus was circulating in Katherine from November 4-13.

“There is a very real possibility that there are people in Katherine who have COVID and either don’t know it or don’t want to know. And those people have then spread it into the community,” Mr Gunner said.

“If that is what’s actually happened, best-case scenario – we got lucky that it hasn’t spread any further than what we have caught so far. Worst-case scenario – this has been silently spreading for a week and we’re a day or two away from discovering a disaster.”

Greater Katherine and Robinson River are under seven-day lockdown orders, which are scheduled to end late Monday.

Late on Thursday, there was a positive waste water sample in the Binjarri Aboriginal community on the outskirts of Katherine. It is within the Katherine lockdown area and home to about 200 people.

NT health authorities aim to test everyone in the community on Friday.

“The only way to get to the bottom out of this is by testing the hell out of the Katherine community,” Mr Gunner said.

All other wastewater testing in the territory has so far been negative.

-with AAP