A tropical depression near Tonga has developed into a cyclone.
The Metservice says tropical cyclone Ian is about 280 kilometres north-northwest of Tongatapu.
Forecasters say the cyclone is intensifying and currently slow moving.
They say tropical cyclone Ian may bring damaging gale force winds over Ha'apai and Vava'u island groups, which can expect heavy rain, squally thunderstorms and very rough to high seas.
There is also a risk of sea flooding of low-lying areas.