A top Defence Force officer has said the Australian navy vessel HMAS Adelaide was not responsible for the emerging COVID-19 outbreak that has sent Tonga into lockdown.

Australia has also promised to help the Pacific nation identify the source of the outbreak by testing samples from the two dock workers who tested positive on Tuesday. 

The number of COVID cases grew from two to five today after three family members of one of those workers also tested positive. 

Tongan authorities feared an influx of international planes and ships delivering water, shelter and food supplies in the wake of the devastating volcanic eruption and tsunami last month, could bring COVID-19 to the nation.

Tonga had recorded only one previous COVID case in October.

HMAS Adelaide — which delivered badly needed supplies to Tonga last week — had around 30 COVID-19 cases on board when it docked at the capital Nuku'alofa.

But Tonga's Prime Minister, Siaosi Sovaleni, said on Tuesday that HMAS Adelaide had no connection with the outbreak, and the two workers who tested positive had no contact with the Australian navy ship or the supplies it delivered. 

Lieutenant General Greg Bilton told Sky News on Wednesday that every precaution was taken to ensure that delivery was safe, and without human-to-human contact. 

"Equally we unloaded in a manner that was COVID-friendly, contactless ... so I don't think there's any connection, there's no evidence of that."

Lieutenant General Bilton said samples from the two workers would be flown back to Australia to "verify the strain" and help Tongan authorities "identify for certain where it might have emanated from." 

Tonga's deputy head of mission in Australia, Curtis Tu'ihalangingie, said the two cases were detected at a wharf used for commercial cargoes

Mr Tu'ihalangingie said delivery of aid would not change as a result of the COVID cases, adding, "frontliners will need to be more careful".

Tongans queued at shops and banks on Wednesday ahead of a lockdown to start at 6pm, as authorities attempt to stop the spread of COVID.

Pastor Akuila Salabogi is a Fijian living in Tonga and said shelves have been stripped bare as people stocked up on supplies before lockdown. 

"People are rushing into the shops and trying to get their shopping done and the shops are crowded ... I could say they're panic buying," he said.

"There is much worry about these [COVID-19] cases, which we heard of in the community."

Health authorities were administering booster shots to the public on Wednesday, with more vaccine booster doses to arrive from Australia and New Zealand.

Around 83 per cent of the eligible population have received two doses of vaccine.

In addition to HMAS Adelaide, three New Zealand and one British naval ships, and two Chinese fishing ships from Fiji have come into port and unloaded aid pallets.

French, Japanese and Chinese navy ships are also en route carrying aid.

The Tongan government has insisted on contactless delivery of aid, and all pallets unloaded from aircraft or ships, are isolated for 72 hours before being distributed by the local emergency service.

At a press conference on Tuesday, Tongan authorities said the two workers at Queen Salote wharf who tested positive were vaccinated, and it was unknown whether they had the Omicron variant.

HMAS Adelaide experienced a power outage while anchored near Tonga, and civilian specialists were being sent from Australia to conduct an assessment, Australia's Department of Defence said.

Back-up power has been activated to restore essential systems, it said.

Two Chinese navy ships left Guangzhou in southern China on Monday carrying humanitarian supplies and shelter, Chinese state media reported.

Two commercial Chinese fishing boats delivered aid to Tonga's Vuna wharf last week from Fiji.

ABC/ Wires