The Tonga Meteorological Services says the tsunami warning is in effect for all of Tonga on Saturday.

Authorities in nearby Fiji also issued a warning, telling people to avoid the shoreline due to strong currents and dangerous waves.

The Islands Business news site reported a convoy of police and military troops evacuated Tonga's King Tupou VI from his palace near the shore. He was among the many residents headed for higher ground.

The explosion of the Hunga Tonga Hunga Ha'apai volcano was the latest in a series of spectacular eruptions.

A Twitter user identified as Dr Faka'iloatonga Taumoefolau posted video showing waves crashing ashore.

"Can literally hear the volcano eruption, sounds pretty violent," he wrote, adding in a later post: "Raining ash and tiny pebbles, darkness blanketing the sky."

Earlier, the Matangi Tonga news site reported that scientists observed massive explosions, thunder and lightning near the volcano after it started erupting early on Friday.

The site said satellite images showed a 5km-wide plume of ash, steam and gas rising up into the air to about 20km.

More than 2300km away in New Zealand, officials were warning of storm surges from the eruption.

The National Emergency Management Agency said some parts of New Zealand could expect "strong and unusual currents and unpredictable surges at the shore following a large volcanic eruption".

The Joint Australian Tsunami Warning Centre said a 1.2m tsunami wave was observed at Nuku Alofa in Tonga at 3.30pm AEDT.

There was no tsunami threat to Australia.

The Pacific Tsunami Warning Center said the threat to American Samoa appeared to have passed, although minor sea fluctuations could continue.

Australian Associated Press