Tonga tsunami alert amid undersea eruption

A file photo of the volcano erupting in 2015
A volcano near Tonga that erupted in 2015 has activated again, triggering a tsunami warning. -AP

Tonga has issued a tsunami warning after an undersea volcano erupted.

Video posted to social media showed large waves washing ashore in coastal areas of the Pacific nation on Saturday.

The Tonga Meteorological Services said a tsunami warning had been put in force for all of Tonga.

The eruption on Saturday was the latest in a series of spectacular eruptions from the Hunga Tonga Hunga Ha'apai volcano.

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 had 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.