Flights between Australia and Tonga should be back up and running by Wednesday after plumes of volcanic ash above the small island nation grounded planes.

Virgin Australia said it would be putting on an additional service from Sydney to Tonga's Fuaamotu International Airport on Wednesday after it was cancelled on Monday.

The airline also had to cancel a flight from Tonga to New Zealand on Tuesday.

"The forecast for tomorrow is looking a lot better so we're confident things will return to normal," a spokeswoman said.

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Fiji Airlines, which also operates between the South Pacific and Australia, is also understood to have cancelled flights.

Air New Zealand said in a statement that a Monday flight from Auckland to Tonga was diverted to Samoa and later returned to Auckland.

It said flights on Tuesday were cancelled but it had scheduled additional flights for later in the week.

Tonga Airports Ltd chief executive Viliami Ma'ake said all flights arriving from and departing to Australia, Fiji and New Zealand had been cancelled.

But he said domestic services were continuing as scheduled.

Brad Scott, a volcanologist at New Zealand agency GNS Science, said volcanic activity has been recorded for several weeks in a stretch of ocean and small islands about 60km north of the capital, Nuku'alofa.

He believes the volcano may have started beneath the ocean's surface and grown until it reached the surface, essentially creating a new island.

But he said details of the eruption still remain unclear.

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-AAP