Tonga has this morning declared a state of emergency as a powerful tropical cyclone heads towards the country.

Acting Prime Minister Semisi Sika issued a statement from the island kingdom as it waits for Tropical Cyclone Gita - nearing a category 4 to 5 strength - which is due to strike later tonight.

Sika said he was satisfied that an emergency is happening or about to happen in the kingdom due to the destructive winds and sea rises caused by the cyclone.

"It is necessary for emergency powers to be exercised in order to prevent or minimise the loss of human life, illness or injury, property loss or damage and damage to the environment...''

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WeatherWatch said the cyclone is forecast to make "a direct hit" over the top of Tonga and the capital, Nuku'alofa, on the main island of Tongatapu late tonight and early hours of tomorrow.

"Over 75,000 people are in the path of a storm already more powerful than anything New Zealand has ever seen, with torrential rain and damaging winds forecast to be averaging close to 200km/h and gusting over 230km/h by tonight when it makes landfall there,'' a spokesman said.

The Red Cross in Tonga is advising the public to be prepared as Cyclone Gita approaches.

The NZ MetService said the cyclone strengthened overnight - meaning the winds in the centre of it could reach up to 167km/h.

"It bypassed Niue overnight, fortunately the winds did too with just strong to gale winds there.

"It is now moving west towards the southern islands in Tonga, and has the potential to strengthen to a category 5 with winds in the centre reaching 200km/h.

"It is likely to pass much of Tonga tonight, and is likely to pass over or near the capital, Nuku'alofa."

The cyclone has already wrought significant damage in both Samoa and American Samoa, and is heading towards Tonga and then southern Fiji.

As those in Samoa take on the mammoth task of cleaning up, those in Tonga are preparing for the worst; with people being told to remain indoors.

New Zealanders in Tonga should be aware that Severe Tropical Cyclone Gita is expected to make landfall on the afternoon...

Posted by New Zealand High Commission, Nuku'alofa, Tonga on Saturday, 10 February 2018

New Zealanders in Tonga are being advised to follow the advice of local authorities at all times - including evacuation orders - and to stay "well away" from the sea due to the possibility of a storm surge.

Kiwis there are also being urged to keep families and friends back home updated about their whereabouts and safety and to register their names on the Ministry of Foreign Affairs and Trade site: SafeTravel.

New Zealanders in Tonga requiring urgent consular assistance may contact the New Zealand High Commission in Tonga on the following number +676 23122 or after hours +676 8817022