An undersea volcano has erupted near the Tongan capital Nuku'alofa, sending plumes of steam and smoke hundreds of metres into the air, officials say.

Tonga's head geologist, Kelepi Mafi, said there was no apparent danger to residents of Nuku'alofa and others living on the main island of Tongatapu.

The eruption, believed to be about 10 to 12 kilometres off the Tongatapu coast, was thought to have started Monday (local time), Mr Mafi said, but it was two days before Nuku'alofa residents first reported seeing the plumes.

Officials said it may be related to a quake with a magnitude of 4.4 which struck last Friday around 35 kilometres from the capital at a depth of nearly 150 kilometres.

An eruption in 2002 in the same area off the western end of Tongatapu, near two small volcanic islands, resulted in an islet appearing for several weeks afterwards.

"This eruption is bigger than in 2002, it seems there are two vents coming out, but that is yet to be confirmed," Mr Mafi said.

Tonga, which lies nearly 2,000 kilometres north-east of New Zealand, sits on the Pacific "Ring of Fire", an area of frequent volcanic and earthquake activity where continental plates in the earth's crust meet.