A massive clean up is under way across Samoa and American Samoa after they bore the brunt of the cyclone at the weekend.
Some parts of Samoa are still underwater as emergency teams work to restore power and running water to thousands of households.
Schools have been closed and there are raised fears about sanitation and the spread of mosquito-borne dengue fever.
The cyclone is now around 450 kilometres east of Tongatapu and moving towards the island.
The Tonga met service said the country should feel the worst of the Cyclone Gita, which is currently a category four storm, around midnight.
The storm is packing winds of over 200km/h and is forecast to intensify.
Tonga MetService director Ofa Fa'anunu said authorities were worried about structural damage from the wind load.
Mr Fa'anunu said because Tongatapu was low-lying, especially on the northern shores, associated storm surges also have people concerned.
"We are worried because it looks like it will hit in the night time as well but heavy rain and flooding is always a problem on the main island with water settling because it is quite flat," he said.
The country is on a high state of alert, with schools and workplaces closed as people make final preparations for tonight's expected impact.
Niwa principal forecaster Chris Brandolino told Morning Report Tonga would face a direct hit or close to it.
He said the storm's centre was forecast to go south of Fiji, though its southern areas would get high winds and heavy rainfall.
"Where it goes after that, I guess that's the $64,000 question."
Red Cross worker based in Suva Hannah Butler told Morning Report locals were well aware that it was tropical cyclone season, and Tongans would be taking extra precautions.
She said people were out chopping down tree branches, removing iron or wood that could become dangerous in high winds, and identifying strong buildings that could become shelters.
"People are going battening down the hatches and this kind of preparedness works, it saves lives."