The modified rotation of US Marines will carry out exercises exclusively at Australian Defence Force training areas in the Northern Territory after undergoing two weeks' quarantine at a Defence base in Darwin.

Defence Minister Linda Reynolds gave approval US Secretary of Defense Mark Esper for the reduced contingent in May.

At the time, she said the approval "was based on Australia's record to date in managing the impacts from COVID-19, as well as strict adherence by deployed US Marines to the mandatory 14-day quarantine and other requirements".

Despite the arrival of US Marines, Australian borders remain closed.

The last confirmed cases of coronavirus attributed to the Northern Territory came on May 1, when two Australian Defence Force personnel, who contracted coronavirus on deployment in the Middle East, were flown to Darwin.