This satellite image shows a cleanup underway on the runway at the Fua’amotu International Airport in Tonga. AFP

Tongans raced on Wednesday to clear a thick coat of ash from the main international runway to let in emergency aid, as the island nation faced a month of crippled international communications following a violent volcanic eruption and tsunami.

The Hunga Tonga-Hunga Ha'apai volcano erupted 30 kilometres (19 miles) into the air on Saturday sending ash, gas and acid rain across a large area of the Pacific.

It released an enormous pressure wave that traversed the planet, travelling at supersonic speed at about 1,231 kilometres per hour (764 miles per hour), New Zealand's National Institute of Water and Atmospheric Research said.

READ MORE

Tsunami-hit Tonga islands suffered extensive damage, fears death toll to rise

Impact assessment, aid efforts underway as world responds to Tonga tsunami

Meanwhile, two New Zealand navy vessels will arrive in Tonga on Friday, carrying much-needed water and other supplies for the Pacific island nation reeling from a volcanic eruption and tsunami, and largely cut off from the outside world.

Hundreds of homes in Tonga's smaller outer islands have been destroyed, and at least three people were killed after Saturday's huge eruption triggered tsunami waves, which rolled over the islands causing what the government has called an unprecedented disaster.

HMNZS Aotearoa departs to provide disaster relief to Tonga from Auckland, New Zealand, on Tuesday. Reuters

With its airport smothered under a layer of volcanic ash and communications badly hampered by the severing of an undersea cable, information on the scale of the devastation has mostly come from reconnaissance aircraft.

"For the people of Tonga, we're heading their way now with a whole lot of water," Simon Griffiths, captain of the HMNZS Aotearoa, said in a release.

In the tsunami that followed, waves in Tonga rose up to 15 metres (50 feet) in some areas, the small Pacific island nation's government said in a statement.

Three people were killed and "a number" were injured, the Tongan government said, calling the volcano explosion "an unprecedented disaster."

The ashfall and tsunami affected more than 100,000 people, virtually the entire Tongan population, the United States Agency for International Development (USAID) said in an update, quoting the Tonga Red Cross Society.

The Royal Australian Air Force and the Australian Defence Force fly along the coastline of Tonga. AFP

Australia and New Zealand have military transport C-130 aircraft ready to fly when ash has been removed from the main island.

Ash particles pose a threat to modern jet aircraft, including by melting and accumulating in the engines.

A thicker-than-expected blanket of five to 10 centimetres (two to four inches) of ash had accumulated on the main island Tongatapu's runway, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said. It expected the runway to be clear soon.

"The ash has proven more difficult to clear than was expected," said Jonathan Veitch, Fiji-based UN coordinator on the crisis.