Tropical Cyclone Gita formed on Tuesday over the Wallis and Fortuna Islands in the southwest Pacific, headed east to the Samoan Islands and is now continuing its clockwise motion and aiming directly for Tonga.
The storm is forecast to be closest to the main island of Tongatapu at around midnight.
Tonga's Fua'amotu Weather Forecasting Centre warned residents could expect "very destructive hurricane force winds".
Red Cross staff checking on people flooded out in Samoa.
Gita slammed into Samoa overnight Friday, forcing the evacuation of some 200 people and causing widespread flooding, leaving many without power and electricity.
With the country already addressing an outbreak of dengue fever in Tongatapu, the deterioration of sanitation conditions on the island could increase the risk of a second disaster due to the spread of dengue and other vector and water-borne diseases.
Cyclone Gita is now a Category 4 but is expected to be a Category 5 when it hits Tonga about 7pm today.
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The storm's impact may also be felt in the low-lying Ha'apai group, where damaging winds, rain and storm surge are likely.
Tonga MetService director Ofa Fa'anunu said authorities were anxious about structural damage from the wind load.
A state of emergency has been declared.
"Where it goes after that, I guess that's the $64,000 question".
"That's an incredibly strong cyclone. people are getting prepared.it really does save lives", said Hanna Butler, Red Cross Pacific spokeswoman based in Fiji.
She said people were out chopping down tree branches, removing iron or wood that could become unsafe in high winds, and identifying strong buildings that could become shelters.