Tropical Cyclone Tino is expected to make a direct hit on the Tonga capital, Nuku'alofa, later on Saturday night.
More than 40 evacuation centres have been opened in the kingdom, and residents are being told to stay indoors.
The storm has already passed Fiji, where disaster officials have reported two people as missing after being swept away in floodwaters.
Pictures taken hours ago on the outer Tongan island of Ha'apai, a popular tourist area for whale watching, show the effect the category 3 storm is having already in the area.
Since then, the torrent generated by Tino has only worsened.
Melino Maka, chairman of the Tonga Advisory Council, says the low-lying island is particularly vulnerable to flooding and people have been taking precautions like nailing down windows.
"[They are] trying to prepare themselves for the unknown," says Maka. "Because at the moment, nobody knows. It's up to the mercy of Mother Nature."
Nuku'alofa, still recovering from Category 4 storm Cyclone Gita in 2018, is facing the prospect of a direct hit.
Some schools are still using tents for classrooms and many homes are yet to be rebuilt following that disaster.
"Some of the schools in the Eastern District are still using the UNICEF tarpaulins," says Maka.
Paula Ma'u, chief executive of the National Emergency Management Office, says residents are urged to find a safe place to weather the storm.
"We are warning people to take precautions and for those that feel they want to, move to evacuation centres," says Ma'u.
Satellite images show the size of the storm, which has winds that could reach 160km/h.
"Priorities of course are lives and also the properties for the damage, any damage caused by the cyclone," says Ma'u.
The tropical cyclone intensified into a category 3 storm after passing through the Fiji island group.
In Fiji, the Disaster Management Office reported a father and daughter missing in Serua, on the main island of Viti Levu.
The effects of the cyclone could also be felt here in New Zealand - Surf Life Saving NZ says waves on the east coast could be much bigger next week.
"These have the power to knock people over and will travel well past the high-tide mark," says Allan Mundy, national lifesaving manager at Surf Life Saving NZ. "Young children, the elderly and small dogs are most at risk."
For the time being, police and Tonga's defence force are on standby to respond to what could be a wild night of destructive winds and water in the kingdom.
