The report from Landcare Research New Zealand states that there are few comprehensive records of coastal inundation events in Tonga, but many low-lying areas have a high exposure to inundation. The most severe inundation in living memory occurred during Cyclone Isaac in 1983 where a storm surge of about 1.6m acted on top of a high spring tide. It was estimated that approximately 30% of Tongatapu inundated (not all of this would have been by seawater – flooding due to heavy rainfall would also have inundated many areas).
The most severe inundation during a tropical cyclone occurred during Isaac in March 1982. On Tongatapu, the passage of the cyclone coincided with a high spring tide, which was about 1.39 m above Cart Datum (1990). The worst affected areas were at Sopu, localized areas to the west and to the east of Manuka. The water level observed across the Vuna Road at Queen Salote wharf was about 0.5 to 0.75 m above the level of the road. All houses fronting the road were moved off their foundations a distance of about 10 m. Based on observed debris lines, the storm tide level reached approximately 3.05 m above Chart Datum resulting in a storm surge was estimated at 1.5 m. In Sopu, water depths were up to 1.5 m but more generally about 1 m in low-lying property behind the coast road in Nuku`alofa. Inundation extended around 300 m inland except at Sopu where it reached 1 km inland. On Ha`apai, the passage of the cyclone coincided with low tide, resulting in little inundation of coastal land.