Tonga will go into lockdown after two fresh cases of Covid were recorded in the capital city Nuku'alofa.
Prime Minister Siaosi Sovaleni said both cases were detected at the port, where humanitarian aid has been arriving following the recent volcanic eruption and tsunami.
The South Pacific nation had previously been virus-free.
So far, all aid deliveries there have been handled using contactless protocols to stop the virus spreading.
In a national address late on Tuesday, Sovaleni said Tonga would enter lockdown from 6PM local time (0500 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."
The remote island nation was one of the few places in the world to escape relatively unscathed from Covid, remaining largely virus-free except for a single case that was detected in October.
But after a volcanic eruption triggered a tsunami that devastated the nation, Tonga has been heavily dependent on foreign humanitarian aid - led by countries like Australia and New Zealand - for supplies of fresh drinking water, shelter kits and rescue equipment.
Tongan authorities had stressed the need for all foreign aid deliveries to be handled using strict "no-contact" protocols in an attempt to keep the virus at bay, including leaving humanitarian supplies in isolation for three days before they are handled by Tongans.
Last week however, a Covid outbreak hit a crucial Australia relief ship bound for the island nation, infecting 23 of the estimated 600 crew members.
The ship eventually docked at the capital's port despite the outbreak. However, Mr Sovaleni did not mention if that was the same ship the affected men had been working with.
At least 60% 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.
