"You can think of it as 'a shotgun blast' straight up into the sky," said marine geologist and Niwa project director Dr Kevin Mackay. "Some of that material went beyond even the stratosphere into the mesosphere (57km in altitude) - the highest recorded eruption column in human history," he told BBC News.
This transport took the form of pyroclastic density currents, which are avalanches of tumbling, scorching rock. In water, their searing heat would have enveloped them in a frictionless steam cushion on which they could simply run and run at very high speed.
This explains, for example, the loss of the submarine cable connecting Tonga to the global internet. A large section was cut out of this data link despite lying 50km to the south of Hunga-Tonga and beyond a large hill on the seafloor.
"Where you had these flows, there is nothing living there today. It's like a desert 70km from the volcano," Dr Mackay said. "And yet, amazingly, just under the rim of the volcano, in places that avoided these density currents, you do find life. You find sponges. They dodged a bullet."
The Niwa team says there were essentially four ways water was displaced to generate these tsunamis: by the density flows pushing the water out of the way; through the explosive force of the eruption also pushing on the water; as a result of the dramatic collapse of the caldera floor (it's dropped by 700m); and by pressure waves from the atmospheric blast acting on the sea surface.
A good example is the biggest wave to hit Tonga's main island, Tongatapu, 65km to the south of Hunga-Tonga. This occurred just over 45 minutes after the first major eruptive blast. A wall of water several metres high washed over the Kanokupolu peninsula, destroying beach resorts in the process.
"For the big local waves - in order to understand them correctly, I believe you also have to have this atmospheric coupling," she explained. "We had a huge pressure anomaly that on its own would have generated a tsunami. So, when you've already got waves, then you're just adding energy to them."
The second stage, directly above the mountain, was given over to the British robot boat USV Maxlimer. Operated by Sea-Kit International from a control room 16,000km away in Tollesbury, UK, this uncrewed vehicle was able to identify ongoing, albeit relatively subdued, volcanic activity. The boat did this by tracing a persistent layer of glassy ash in the caldera back to its source - a new vent cone some 200m under water.