An underwater volcano has erupted in the South Pacific with a stunning blast, sending tsunami waves onto the nearby island of Tonga and to the north in Japan, with warnings of dangerous ocean surges issued as far away as the US West Coast.

Dramatic satellite images showed the long, rumbling eruption of the Hunga Tonga-Hunga Ha'apai send a huge mushroom of smoke and ash into the air and a shockwave across the surrounding waters.

A tsunami wave measuring 1.2 metres was observed in Tonga's capital Nuku'alofa, according to Australia's Bureau of Meteorology.

The Japan Meteorological Agency said a tsunami reached that country's Pacific coast, too, with waves as high as three meters possible.

A 1.2-metre wave reached the remote southern island of Amami Oshima and other areas along Japan's Pacific coast observed smaller surges, the agency said.

People scrambled to higher ground on the tiny island of Tonga. Local resident Mere Taufa said she was in her house getting ready for dinner when the undersea volcano erupted - sending water crashing into her home.

"It was massive, the ground shook, our house was shaking. It came in waves. My younger brother thought bombs were exploding nearby," Ms Taufa told the Stuff news website.

She said water filled their home minutes later and she saw the wall of a neighbouring house collapse.

"We just knew straight away it was a tsunami. Just water gushing into our home.

"You could just hear screams everywhere, people screaming for safety, for everyone to get to higher ground."

Tonga's King Tupou VI was reported to have been evacuated from the Royal Palace in Nuku'alofa and taken by police convoy to a villa well away from the coastline.

The volcano's eruption lasted at least eight minutes and sent plumes of gas, ash and smoke several kilometres into the air.

Residents in coastal areas were urged to head for higher ground following the eruption -- which came just a few hours after a previous tsunami warning was lifted on the island.

The Hunga Tonga-Hunga Ha'apai volcano sits about 65km north of the Tongan capital Nuku'alofa.

Its latest eruption was so intense it was heard as "loud thunder sounds" in Fiji more than 800km away, according to officials in Suva City - where images shared on social media showed large waves hitting the coast.

Tsunami warnings were issued for American Samoa, New Zealand, Fiji, Vanuatu, Chile and Australia - where authorities said a swathe of coastline, including Sydney, could be hit by tsunami waves.

Jese Tuisinu, a television reporter at Fiji One, posted a video on Twitter showing large waves washing ashore, with people trying to flee from the oncoming waves in their cars.

"It is literally dark in parts of Tonga and people are rushing to safety following the eruption," he said.

People in surrounding New South Wales state were "advised to get out of the water and move away from the immediate water's edge".

A tsunami warning was issued for the entire US West Coast - from the bottom of California to the tip of Alaska's Aleutian islands - while tsunami waves triggered "minor flooding" in Hawaii according to the Pacific Tsunami Warning Center.

"A Tsunami is occurring. Remember: the first wave may not be that largest. Move away from the shore and head to high ground," the US National Tsunami Warning Center wrote.

Footage shared on social media appeared to show a wave of about a foot washing into a coastal inlet in the state of Oregon.

Canada issued a tsunami advisory for British Columbia province and urged people to stay away from beaches and marinas.

Fijian officials warned residents to cover water collection tanks in case of acidic rain fall.

Victorina Kioa of the Tonga Public Service Commission said Friday that people should "keep away from areas of warning which are low-lying coastal areas, reefs and beaches".

And the head of Tonga Geological Services, Taaniela Kula, urged people to stay indoors, wear a mask if they were outside and cover rainwater reservoirs and rainwater harvesting systems.