Tens of thousands of people on Tonga's main island are preparing for the worst as Cyclone Gita bears down with category four force.

Key points: Authorities say there will be a high danger period of 24 hours

Authorities say there will be a high danger period of 24 hours Winds gusts already reaching 230kph

Winds gusts already reaching 230kph Samoa declares state of emergency

Bringing destructive winds and storm surges, Gita is expected to start affecting the island Tongatapu, where more than 70,000 people live, on Monday afternoon.

Tonga's National Emergency Management Office said it was still calm in the capital but that was expected to change soon.

"From our end we are alerting people to be prepared," director of Tonga's National Emergency Management Office Leveni Aho said.

"It's still pretty calm in Nukualofa at the moment, but it's giving us every opportunity to prepare.

"We are in the process of informing everyone to prepare for the worst."

Cyclone Gita hit Samoa over the weekend. ( Twitter: IFRC Asia Pacific )

Authorities said Gita was expected to develop into a category five cyclone and was already packing winds of 165 kilometres per hour with gusts of 230kph.

A state of emergency has been declared and Mr Aho said there would be a high danger period of about 24 hours.

"About 1 o'clock today to 1 o'clock tomorrow afternoon," he said.

"We're advising people to stay tuned to their radios."

Cyclone Gita leaves Apia under water

There was severe flooding in Samoa's capital Apia. ( Twitter: @alvaro_undp )

Neighbouring countries American Samoa and Samoa suffered significant damage over the weekend with Samoa declaring a state of emergency.

The cyclone hit early on Saturday, causing widespread flooding and forcing about 300 people in Samoa to evacuate their homes.

Much of the capital, Apia, was left under water and schools were still closed as the clean-up continued.

Samoa's Red Cross said people were well prepared after learning lessons from a devastating cyclone five years ago.

"It is actually a repetition of what happened during Cyclone Evan in 2012," Samoa Red Cross secretary-general Tautala Mauala said.

"There has been heavy flooding, especially in central Apia.

"The same families that were affected [last time] are again affected from this flooding."