Tropical cyclone Ula is passing near the northern island of Vava'u as the government has declared a state of emergency.
Cyclone Ula has been bringing heavy rain to northern Tonga as it is pushing westward and away from Vava'u.
The category 3 system has destructive winds of 150 kilometres an hour with gusts up to 200 kilometres an hour.
A hurricane warning for Vava'u has been cancelled but Laitia Fifita of Tonga's Met Service on Tongatapu says seas will continue to be very rough.
"It is expected that the speed of movement of the tropical cyclone will speed up a little bit, so at around midday today it is expected to lie further west out of from the island of Vava'u and continue on a south southwest direction."
Liatia Fifita says the weather station in Vava'u has reported heavy rain and damage to vegetation.
Last night, the Prime Minister Akilisi Pohiva declared an emergency, saying it was in order to prevent or minimise the loss of human life, illness or injury, property loss or damage and damage to the environment.
The cyclone is expected to weaken when it reaches Fiji's Lau group by tomorrow.
Strong wind warnings extend for much of the South Pacific, including the Cook Islands, Niue and Tuvalu.