The two cases are local port workers who had been helping to distribute aid received after a volcanic eruption and tsunami last month.

The disaster polluted drinking water, severed communications, killed three people, and left dozens homeless.

Australia, New Zealand, Japan, the UK, and China all offered to deliver aid, but steps were put in place to try to make sure they did not also bring COVID-19.

Tonga had previously reported just one case of the virus - a missionary returning from Africa via New Zealand in October. Arrivals are usually required to spend three weeks in quarantine.

The countries delivering aid promised to bring fresh water and medicines without coming into contact with Tongans on the ground.

But dozens of sailors on Australian aid ship HMAS Adelaide became infected, as did crew on flights from Japan and Australia.

The two positive Tongan cases were revealed after 50 frontline workers at the port were tested, according to news website Matangi Tonga.

The lockdown began on Wednesday evening (Wednesday morning UK time) and its duration has not been confirmed.

Health officials are expected to update the situation every two days.

Tonga and other small Pacific nations such as Kiribati and the Solomon Islands were among the last countries to see COVID-19, thanks to their remoteness and strict border controls.

According to Our World In Data, about 61% of Tongans are fully-vaccinated.