New Zealand's government has confirmed two deaths on volcano-ravaged Tonga and postponed delivery of relief supplies due to volcanic ash on an airport runway.

The full extent of damage from Saturday's massive volcanic eruption remains unknown, as communication with the island country has been disrupted.

New Zealand's government on Tuesday said Tongan police reported that two people had died.

One of the victims was a Tongan national, and the other a 50-year-old British woman who was swept away by a tsunami.

Australia and New Zealand sent military patrol aircraft to Tonga on Monday to survey the damage.

Reconnaissance images show the runway and homes covered with ash, and toppled containers at a seaport.

New Zealand had planned to deliver drinking water and other relief supplies on Tuesday using military aircraft, but postponed the mission to Wednesday at the earliest as the runway must be cleared.

In Japan, Chief Cabinet Secretary Matsuno Hirokazu said Tokyo received an official request for assistance from Tonga's government on Tuesday, through the Japanese Embassy in the country.

Matsuno said Japan's government will work closely with Tonga and other countries to decide what to send and means of delivery, and offer aid as quickly as possible.

He added that Japan has been sharing information with Australia and New Zealand on the situation in Tonga, and making arrangements for recovery and reconstruction support.