Forecasters say tropical cyclone Ian is likely to be upgraded to a Category 4 storm by tomorrow morning and could cause damage to the Tongan island groups of Vava'u and Ha'apai.
The cyclone is currently a Category 3, with estimated winds of 120 kilometres an hour at its centre with momentary gusts of more than 165 kilometres an hour.
The Fiji Meterological Service acting director, Aminiasi Tuidraki, says it is a possibility the cyclone's centre will not reach land and pass to the south of Vava'u.
But he says gale force winds, thunderstorms and flooding of low lying coastal areas are expected.
"The wind force, when it reaches Tonga, is going to be Category 4 some time from tomorrow morning. So we have issued already the alert for the Vava'u and Ha'apai area, and once it's closer we will issue a tropical cyclone warning for those areas."
Aminiasi Tuidraki says strong wind warnings for the Niuas island group also remain in force.