Underwater Tonga volcano triggers alerts

Hunga Tonga-Hunga Ha'apai volcano near Tonga
Satellite images showed a huge eruption from the Hunga Tonga-Hunga Ha'apai volcano near Tonga. -PR Handout Image

Tsunami alerts have been issued across large parts of the South Pacific after an underwater volcano near Tonga erupted and sent large waves crashing ashore on the island kingdom.

The giant eruption of the Hunga-Tonga-Hunga-Ha'apai volcano on Saturday triggered a tsunami in Tonga.

Local media on the island chain reported a downpour of ash, the flooding of properties and disruptions to phone and power connections but the extent of the devastation was unclear by late evening.

The Bureau of Meteorology in Australia said a 1.2 metre tsunami wave had been observed in Nuku'alofa, Tonga's capital.

The Islands Business news site reported that a convoy of police and military troops moved Tonga's King Tupou VI from his palace near the shore.

He was among the many residents who headed for higher ground.

Satellite images showed a huge eruption, a plume of ash, steam and gas rising like a mushroom above the blue Pacific waters.

The New Zealand Defence Force said it was monitoring the situation in Tonga and stood ready to assist if requested by the government but that so far no request had been made, according to Radio New Zealand.

A spokesperson for the Australian government said initial assessments are underway and the Department of Foreign Affairs is working to ensure Australians in Tonga are safe and accounted for.

"Tonga is part of our Pacific family and our thoughts are with the entire community dealing with the impact of the volcanic eruption and tsunami," the spokesperson said in a statement.

"The Prime Minister and Minister for Foreign Affairs are monitoring the situation and Australia stands ready to provide support to Tonga if requested."

Authorities across the Pacific, including in Samoa, Fiji and New Zealand, issued tsunami alerts, warning people to stay away from coastal areas due to the possibility of strong and unusual currents and unpredictable surges.

"People in or near the sea should move out of the water, off beaches and shore areas and away from harbours, rivers and estuaries," the New Zealand National Emergency Management Agency warned.

Hauraki Gulf Weather recorded a 71 centimetre wave at Great Barrier Island, near Auckland following the eruption.

The volcano is located about 2000 kilometres northeast of New Zealand.

New Zealand's MetService said its instruments detected a pressure surge from the eruption. 

New Zealanders took to social media to report they could hear the eruption.

Residents of American Samoa were alerted to a tsunami warning by local broadcasters as well as church bells that rang territory-wide as an outdoor siren warning system was out of service.

As night fell, there were no reports of any damage and the US-based Pacific Tsunami Warning Center canceled the alert.

The Japan Meteorological Agency said there may be a slight swelling of the water along the Japanese coasts but it was not expected to cause any damage.

The volcano also erupted on Friday, when small tsunami waves of up to 30cm were recorded, the Tonga Meteorological Office said.

The volcano was erupting intermittently in late December but Friday's eruption was about seven times more powerful than the previous eruption, Tonga Geological Services said.

with reporting from AP