Up to 70% of the country's population is at risk with the severe tropical cyclone expected to pass near the populated main islands of Tongatapu and 'Eua.
Some parts of Samoa are still underwater as emergency teams work to restore power and running water to thousands of households.
Acting Prime Minister Semisi Sika issued a nationwide alert, saying he was "satisfied that an emergency is happening or is about to happen".
According to the Fiji Meteorological Service, Tropical Cyclone Gita is forecast to arrive in Tonga on Monday, 12 February 2018 in the evening.
Truckloads of troops were helping people batten down as the capital prepared for the cyclone and evacuation centres were opened across the kingdom.
Oxfam's director of programmes in Tonga, Jane Foster, said the organisation had a long-established presence in Tonga and Samoa and therefore have solid pre-positioned emergency supplied already in-country ready to be deployed when required. Many models are predicting it would hit New Zealand, but not until the middle of next week.
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"It's pretty rare to see the ideal circle, the centre of that storm, going right over the top of such a small island".
For Ono-i-Lau and Vatoa and Southern Lau - expect strong winds and winds may increase to damaging gale force from late tonight to early tomorrow and further increase to destructive hurricane force winds of upto 230 kilometres per hour.
It has strengthened overnight and is forecast to strike Tonga as a category 4 and possibly 5 strength storm. About 300 people had evacuated but were now returning to their homes, Butler said.
Tonga MetService director Ofa Fa'anunu said authorities were anxious about structural damage from the wind load.
Cyclones are common in the Pacific at this time of year, with Category Five systems proving highly destructive when they make landfall.
The TVNZ weatherman has the latest on the storm set to hit the Pacific nation.