The advisory warns the cyclone is expected to hit as a Category 4 storm, bringing destructive winds, heavy rainfall, risky storm surge and flooding to Tonga.
The cyclone has already created havoc in neighbouring Samoa and is threatening to become a Category Five superstorm as it approaches Tonga.
The cyclone would be stronger than Cyclone Fehi, which caused extensive damage on New Zealand's West Coast last weekend.
Maximum wave heights could be in excess of over 10 metres with storm surge at the centre of the cyclone lifting the sea up even further leading to coastal inundation and sending those waves even further inland.
The latest update from the Fiji Meteorological Service showed Gita had strengthened to category 4.
Destructive winds are likely to begin several hours before the cyclone passes overhead or nearby. The Pacific Humanitarian Team, through its partners, stands ready to support the people and governments of the countries affected by Tropical Cyclone Gita should a request be made for worldwide assistance.
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Tropical Cyclone Gita passed by Samoa on 10 February 2018 and Niue on 11 February 2018, with damages reported in Samoa, including localized flooding.
It's expected to grow into a category-three tropical storm during the westward movement.
Average wind speeds close to the centre were expected to be over 200kmh by 7pm on Monday. "And the possibility is very high to intensify along the way up to our area, but we are still monitoring the movement and strength".
For other parts of the country, expect strong winds, heavy rain and thunderstorms from early tomorrow.
The information ministry advised anyone considering moving to the shelters to leave early, saying travel would not be possible when the eye of the storm passes over.
Cyclone Gita's arrival comes as the country is already dealing with another situation; after declaring an outbreak of dengue fever just over a week ago.