Tonga's National disaster office says the massive earthquake which hit the kingdom early this morning is likely to be the biggest on record for the country.
Emergency services were put on full alert following the earthquake measuring 7.8 on the Richter scale.
The US monitoring centre says the epicentre was 155 kilometres south of Neiafu island and 160 kilometres north-east of Nuku'Alofa, the main island.
The deputy director of the disaster office, Male'u Takai, says preliminary assessments in the islands have brought no reports of injury or damage to buildings although the power supply in Tongatapu was disrupted.
"There was an after-shock, but it was soon after the main shock-wave. It wasn't as big as the original one, but it's one that perhaps, I think, is the strongest one on record."