Tropical Cyclone Ian, a category four system packing winds of up to 230 kilometres per hour (143 mph), was expected to make landfall early Saturday, the UN Office for the Coordination of Humanitarian Affairs (OCHA) said.
"The cyclone may cause destructive winds, heavy rain, high swells and flash flooding in low-lying coastal areas," it said, adding that many people were boarding up windows and cutting back trees to minimise potential damage.
Red Cross emergency teams were on standby.
Tonga's National Emergency Operations Committee advised people to stock up on enough food and water to last for three days.
Tonga's 105,000 residents are used to dealing with cyclones, which are common in the Pacific during the November-April hurricane season.
Cyclone Ian has been lying offshore between Fiji and Tonga in recent days as a category three storm but has intensified as it approaches land.
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