Cyclone Winston is about 200 kilometres northwest of Tonga's capital, Nuku'alofa, and slowly moving northeast towards the island of Tofua in the Ha'apai group.
The cyclone remains a category two, with winds as high as 130 kilometres an hour at its centre.
Tonga's Meteorological Service said damaging winds, heavy rains and rough seas could be expected for Ha'apai, Tongatapu, 'Eua and the Niuas.
In Vava'u the winds were predicted to be destructive.
Forecasts show Winston maintaining its category two strength as it crosses Tongan waters, before taking a sharp turn back west towards Fiji some time tomorrow.