A freak storm -- that dumped more than a month's average rainfall in less than 24 hours -- has swamped the Pacific island nation of Tonga, forcing evacuations, officials said Saturday.

Buildings, including the Australian High Commission, and houses in the main city of Nuku'alofa were flooded after roads turned into rivers during the storm, officials said.

"This is the greatest rainfall we have ever had in the kingdom," duty forecaster 'Ofa Taumoepeau said.

Climatologists called it "an extreme event" with up to 289.2 millimetres (11.38 inches) of rain falling on parts of the main island of Tongatapu in the 24 hours to 10:00am Saturday.

Most of the rain fell in a seven-hour period on Friday evening causing flash-flooding across the island and forcing animals to swim for their lives.

The Tongan Defence Force's quick reaction team was called out and evacuated several families who had not been able to reach higher ground.

They also assisted the Australian High Commission move furniture and equipment after the building started to flood.

The water was knee deep in some areas, second lieutenant Esu Tupou said.

"Cars couldn't work and they found some cars stuck without drivers. They came across pigs and dogs swimming in the water."

Australian High Commissioner Bruce Hunt said the back yard of the property flooded and water seeped into the lower level of the immigration and consular offices.

Although the weather was fine over Tonga on Saturday, further heavy rain was forecast for later in the weekend.

Tonga, with a population of 112,000, lies a third of the way between New Zealand and Hawaii and receives most of its annual rainfall between November and April.

A heavier than usual rainy period had been forecast as "a high probability" this year for January to March.