Forecasters say tropical cyclone Ian's centre is likely to hit the Vava'u island group in Tonga tomorrow night after 9 o'clock local time.
The Fiji Meterological Service says the cyclone has been upgraded to a Category 3 storm.
It has estimated winds of 120 kilometres an hour at its centre with momentary gusts of more than 165 kilometres an hour.
The Metservice says strong wind warnings for Ha'apai and Niuas island groups also remain in force.
Thunderstorms are expected over these groups from later today, and flooding of low lying coastal areas is expected from tomorrow.
