The soldier, who does not have any symptoms, arrived on a charter flight on Wednesday and had no direct contact with the general community.

In a statement, the Australian Defence Force says all US personnel who arrived or interacted with the marine would continue to be monitored during their mandatory 14-day quarantine.

NT Health Minister Natasha Fyles says the marine will be taken to hospital for the isolation period.

"But this is not someone who has been mixing in our community," she said.

"This is not a case of community transmission. They have been in quarantine and there is a very low risk to the community."

The infection takes the number of confirmed COVID-19 cases in the Territory to 32.

Ms Fyles said the marine rotations were an important part of Australia's defence operations and had been going on for many years.

"We agreed to host these rotations but we've put measures in place to protect Territorians.

"The system has worked here."

However, the minister said the NT government would work with the ADF to see if any changes to the arrangements needed to be put in place.

She said all the US soldiers who arrived this week were screened before travelling to Australia and would not be not permitted to mix with the wider community.

Chief Health Officer Hugh Heggie said the incident had been handled appropriately.

He said it was possible the individual could have contracted the virus before travelling to Australia and had returned a negative test at that time.

Dr Heggie said US officials and Defence had good plans in place to make sure there was no risk to other people.

Australian Associated Press