Tonga will go into Covid-19 lockdown Wednesday after the virus was detected in a fresh blow to the Pacific kingdom as it struggles to recover from last month's devastating volcanic eruption, officials said.

The remote island nation had been virus free, but Prime Minister, Siaosi Sovaleni said two men tested positive this week in Nuku'alofa.

He said the men had been working in the capital's port, where humanitarian aid has been pouring in from around the globe since the January 15 eruption.

In a national address late Tuesday, Sovaleni said Tonga would enter lockdown from 6:00pm (0500 GMT) Wednesday, with the situation reviewed every 48 hours.

The volcanic blast, one of the biggest recorded in decades, generated massive tsunami waves and blanketed the island nation in toxic ash, claiming three lives.

Tonga closed its borders in early 2020 as the coronavirus pandemic swept the globe.

Since then, the nation of 100,000 had recorded just one case of Covid-19, a man who returned from New Zealand in October last year and has since fully recovered.

However, the devastating blast from the Hunga Tonga-Hunga Ha'apai volcano, which lies about 65 kilometres (40 miles) north of the capital Nuku'alofa, created what the Tongan government describes as an "unprecedented disaster".

Australia's HMAS Adelaide sits near Nuku'alofa to deliver humanitarian stores and medical supplies to Tonga residents
Australia's HMAS Adelaide sits near Nuku'alofa to deliver humanitarian stores and medical supplies to Tonga residents Australian Defence Force via AFP / POIS Christopher Szumlanski

In response, navy ships from Australia, New Zealand, the United States, France and Britain have delivered aid including drinking water, medical supplies and engineering equipment.

All of the deliveries have been handled using strict "no-contact" protocols in a bid to keep the virus at bay.

Sovaleni did not reveal which ship the affected men had been working with.

He said they were asymptomatic and double vaccinated, along with about 85 percent of Tonga's population.

Australia's HMAS Adelaide docked in Nuku'alofa to unload supplies last week, despite a Covid-19 outbreak that infected more than 20 of its crew.

A UN update late last week said drinking water remained the main challenge facing Tonga and about 1,500 people were still displaced.

Communications remain patchy after the eruption damaged an undersea cable that connects Tonga to the rest of the world.

Officials said a specialist cable repair ship was expected to arrive this week and would take at least two weeks to fix the damage.