The volcanic eruption near Tonga shredded an 80-kilometre (50 mile) stretch of undersea cable, complicating efforts to reconnect the Pacific kingdom after a month of digital darkness, the company overseeing repairs says.

Tonga Cable Limited chief executive James Panuve said a repair ship had located the severed ends of the 840 kilometre-long cable linking Tonga to Fiji that was cut in the January 15 blast.

But rather than a clean break, Panuve said the ship found the eruption tore an 80 kilometre section of cable into numerous pieces as it pummelled the seabed with the explosive force of a nuclear bomb.

"It is obvious that the eruption, shockwaves, (and) tsunami caused major havoc underwater," he said Tuesday.

The eruption of the Hunga Tonga-Hunga Ha'apai volcano generated large tsunami waves and blanketed Tonga in toxic ash, claiming three lives.

Disaster relief efforts in the island nation of about 100,000 have been complicated by poor communications, with backup satellite links providing patchy broadband as the operation to repair the cable proceeds.

Satellite image showing cloud from the volcano that erupted in Tonga on January 15.
Satellite image showing cloud from the volcano that erupted in Tonga on January 15. AFP / Laurence CHU

Panuve said that after contending with poor weather last week, the cable repair ship "Reliance" was now trying to retrieve sections of cable in waters up to 2.5 kilometres deep.

He said one section had been moved five kilometres by the undersea blast and another was buried under 30 centimetres of silt.

Despite the difficulties, Panuve was optimistic Tonga Cable could still meet its goal of repairing the digital link by February 20, saying plans were underway to create a "mini-system" to bridge the 80 kilometre gap.

Panuve said a piece of cable more than 55 kilometres long was still lost on the seabed and the ship was searching for it in the hope it was still usable.

In addition to the damage caused by the eruption and subsequent tsunamis, Tonga is also dealing with a Covid-19 outbreak after remaining virus free for much of the pandemic.

The main island Tongatapu and outlying Vava'u remain under lockdown with health authorities on Tuesday reporting a total of 139 cases nationwide.

International humanitarian aid continues to arrive, with two Chinese naval vessels this week delivering 1,300 tonnes of supplies, including 500 prefabricated houses.