Underwater volcano Hunga-Tonga-Hunga-Ha'apai has erupted again sending ash, steam and gas 20 kilometres into the air.

There is a tsunami warning in place for all the islands of the Kingdom of Tonga as the volcanic activity affects coastal water levels - with people advised to stay out of the water and away from the coast.

People on all islands in Tonga are advised to stay out of the water and away from the coast.

There are no warnings in place for Fiji, Samoa or New Zealand.

The maximum tsunami wave has been recorded in Nuku'alofa tide gauge at 12:30pm and was about 30cm and the fluctuation of the sea level is currently observed.

The volcano is located about 30 kilometres south-east of Fonuafo'ou island in Tonga.

Tonga Geological Services head Taaniela Kula said the eruption had a radius of 260km.

The eruption was about seven times more powerful than the last eruption on 20 December last year and continuing to grow, Kula said.

Kula said people on the islands of Tongatapu, the islands of Ha'apai and Vava'u group needed to stay indoors as much as possible, wear a mask if they were outside and cover rainwater reservoirs and rainwater harvesting systems

Changes to seawater levels have also been detected on the south coast of Samoa as a result of this morning's eruption.

Ash fall had been witnessed in the islands of Ha'apai, domestic flights and were on hold at the moment as a result, Kula said.

The volcano was erupting intermittently in late December.