The director of Tonga's emergency management office says authorities are keeping a close eye as Cyclone Winston moves close to the northern Vava'u group and nearby Niue.
Early this morning Cyclone Winston was located about 230 kilometres south of Niuatoputapu, or 80 kilometres north-northeast of Neiafu - moving at about 23 kilometres an hour, with gusts of up to 150 kilometres an hour.
The category two cyclone has already caused some minor damage and flooding of low-lying areas in Tongatapu and Ha'apai.
A staff member at the National Emergency Management Office says a team will conduct a survey of the islands this morning to determine if any further damage was caused overnight.
Its understood power has been lost for most of the islands, although the main hospital remained operational by generator. The office had not had any reports of injuries or casualties as a result of the cyclone overnight.
In its latest special weather bulletin, the Tonga Meteorological Service said strong wind, heavy rain, rough seas, and flash flood warnings remained in force for Tongatapu, 'Eua, Ha'apai, Nuiafo'ou, Niuatoputapu and Vava'u.
The National Emergency Management Office, Leveni 'Aho, said the cyclone was moving towards the warmer waters in Vava'u, in the country's north, and there were fears that would cause it to intensify over the next day or so.
He said the cyclone would pass closer to Vava'u than anywhere else, and there were fears the damage there may be more severe.
He said people in Vava'u should secure their houses and be prepared for Winston's arrival.
To the east, Niue issued its first tropical cyclone alert on Wednesday morning, cyclone Winston was located about 360 kilometres west-northwest of Niue.
It is expected to bring damaging gale force winds to the country over the next 24 to 36 hours.
Forecasters are warning of gale force winds of up to 100 kilometres per hour, flooding, and damaging sea swells.
The cyclone was expected to continue to move towards Niue with the same intensity, before sharply turning back west towards Fiji some time on Wednesday or Thursday.