The Defence Department has confirmed that HMAS Adelaide, which docked in the Pacific island nation last week, had lost power during the operation.

A defence spokesman said back-up power had been activated aboard the vessel to restore essential systems.

"The situation is being closely monitored and the safety of the ship and the embarked forces remains our highest priority," the spokesman said in a statement.

"The power outage has not affected Australia's ability to support requests from the government of Tonga to assist their recovery efforts."

The ship departed from Brisbane and arrived in Tonga last week, carrying critical relief including medical and humanitarian supplies as well as helicopters, following the January 15 underwater volcanic eruption and tsunami.

However, Australian relief efforts were earlier hampered when 23 crew members aboard the ship tested positive to COVID-19, despite rigorous screening measures before departure.

Strict border measures are in place in COVID-free Tonga, with the ship granted permission to dock and offload supplies.

Defence said civilian specialists are on route to conduct an assessment of the affected systems of the HMAS Adelaide, following the loss of power.

The department has denied reports that the ship was being towed or that systems such as air conditioning, sanitation and refrigeration were down due to the outage.

Australian Associated Press