Tonga will go into lockdown after several cases of Covid were recorded in the capital city Nuku’alofa.
Prime Minister Siaosi Sovaleni said on Tuesday two port workers had tested positive. Officials later confirmed three more cases in family members.
The South Pacific nation had previously managed to stay virus-free.
The outbreak comes as Tongans try to recover from a deadly volcanic eruption and tsunami which left three dead and damaged homes and infrastructure.
Tonga had avoided Covid outbreaks by closing its borders to the outside world in early 2020.
But since the eruption it has since been heavily dependent on foreign aid for supplies of fresh drinking water, shelter kits and rescue equipment.
So far, foreign aid deliveries there have been handled using contactless protocols to stop the virus spreading from abroad.
They include leaving humanitarian supplies in isolation for three days before they are handled by Tongans.
Australia, New Zealand, the United States, China, France, Fiji and the UK have all sent ships carrying supplies.
Last week however, a Covid outbreak hit the HMAS Adelaide – a crucial Australian relief ship bound for the island nation – with dozens of crew members infected.
The ship eventually docked at the capital’s port. The Tongan government is investigating but says it does not believe there is a link to the vessel.
The Australian Defence Force’s operations chief said on Wednesday workers who tested positive had been working in a different area of the port to where the warship was and said there was “no evidence” the cases were linked.
“We unloaded in a manner that was Covid-friendly, contactless, in line with arrangements made with Tongan officials at the wharf,” Lieutenant General Greg Bilton told Sky News Australia.
The ship will take back samples from the Tongan cases so that an Australian health facility can assess the strain and find out which country it came from.
In a national address late on Tuesday, Mr Sovaleni confirmed the Covid cases and said Tonga would enter lockdown from 18:00 local time (05:00 GMT) Wednesday, with the situation reviewed every 48 hours.
“The most important issue at the moment is to slow down and stop those who have been affected,” he said, adding that “no boat will be allowed to go from one island to another, no more (domestic) aeroplane flights”.
At least 83% of Tonga’s 106,000 strong population have received double doses of the Covid vaccine. However, the remoteness of some of these island communities, many with limited healthcare resources, makes them particularly vulnerable to an outbreak.
Drew Havea, chair of the Civil Society Forum of Tonga, said: “We assume the worst, that there are going to be more.
“I think the hope now is that we all stick together and find out how big this thing is for Tonga,” he told the BBC from Nuku’alofa.