Five days after one of the largest volcanic eruptions in the last 30 years, new details about the destructive tsunami triggered by Tonga's Hunga-Tonga-Hunga-Ha'apai volcano are emerging, as is the extent of the damage.

The Tongan government's first official update stated that the eruption generated waves of up to 15 meters, which struck the west coasts of a series of islands.

While 15 meters may seem like an enormous height, GNS seismologist Bill Fry believes the waves splashed up to that height in some places rather than rushing in as a 15m high wall of water everywhere.

"It is entirely possible that the 15m mark represents the maximum height reached inland by the water as it came in and sloshed around. It possessed considerable force. Consider throwing a bucket of water out onto a deck; if it hits a wall or your fence, it may splash a little.

"The same type of thing can happen with a tsunami, and I expect that 15m is probably the maximum extent at which it splashed up."

Fry stated that it would not have reached 15m in all locations.

"When we look at point measurements of wave amplitude or wave height in conjunction with a coastal run-up, we always see something that is both the effect of the incoming tsunami - the tsunami that was generated - and the local impacts of the coastline, the shape of the coastline.

"Does the coastline resemble a funnel that concentrates all of the energy in one location, thereby making it more focused and resulting in higher run-ups? Or does it appear to be a shape that disperses the energy and has much smaller height waves - waves with a smaller amplitude?

"So I think that 15m measurement was probably the former case, where there is some sort of local topography that funnelled the energy to a particular place and made it slosh up to that 15m."

Foreign Minister Nanaia Mahuta said yesterday that Tonga's outer islands had sustained significant damage.

Residents of some of Ha'apai's smaller islands began evacuations Wednesday morning, with approximately 150 people evacuated to other islands, Mahuta said.

The Tongan government reported that Mango, Atat, and Fonoifua were evacuated and that multiple people were injured.

Additionally, residents were evacuated from the western side of Tongatapu, including Kanokupolu, where dozens of homes sustained damage.

On two islands, all structures have been destroyed: on Mango, all structures have been destroyed; aerial images from a New Zealand Defence Force flight show only a few temporary tarpaulin shelters remaining standing.

And it appears as though nearly every structure on Atat has been destroyed. After surveillance flights, the New Zealand Defence Force described the damage as "catastrophic."

Additionally, the United Nations Institute for Training and Research (UNITAR) released an initial assessment indicating that 72 structures on Atat had been damaged and the entire island had been covered in ash.