The volcano's remnants stretch across much of the southern part of the area and cover the whole of Hong Kong Island. The centre of its core is directly below the city's CBD.
Dr Sewell, a University of Canterbury-trained geologist who now heads Hong Kong's Geotechnical Engineering Office research team, compared the size of the blast to that at Taupo 27,000 years ago - the world's most recent supervolcano eruption.
The size and composition of the two volcanoes were similar, as was the huge volume of ash and rock they spewed forth.
Taupo's catastrophic eruption sent about 1150cu/km of material into the environment, and the Hong Kong event produced about 1300cu/km.
That was about 500 times more than the eruption of Mt Pinatubo in the Philippines 21 years ago, which was to cool the world's temperature by about 0.4C.
"Such a colossal eruption would have had local and global effects and we think because it was very active in the region, there were probably other types of supervolcanoes going off," Dr Sewell said.
Because the eruption occurred on the boundary between the cretaceous and jurassic periods he believed it might have wiped out some terrestrial species of dinosaur. "It's a compelling justification for perhaps the jurassic-cretaceous period itself."
Surveyors had been studying Hong Kong's geology for years, but it was a striking match between its volcanic rock and granite equivalent that led to the breakthrough.
"The connection between the magma chamber and the volcanic rock convinced us we are looking at a single, very large magmatic system, and when we did the volume calculations of the erupted material, we found it was on a super scale," Dr Sewell said.
That "a-ha moment" came in 2008.
The past four years had been spent proving it. "We couldn't quite believe it when we saw it."
The New Zealander is now convinced more supervolcanoes are waiting to be discovered around the south China region.
"We suspect this is just a microcosm of what is happening on a broader scale.
"There could be other gigantic volcanoes lurking - it's just a case of going and looking for them."
Q&A: Volcanoes
What are supervolcanoes?
The US Geological Survey defines them as having produced an eruption reaching magnitude 8 on the volcano explosivity index, meaning more than 1000 cubic km of magma has been erupted.
None of these enormous eruptions has occurred in recorded history, and their effects would be catastrophic on a global scale. There are about 50 known supervolcanoes in the world.
Are there any in New Zealand?
New Zealand is home to the Taupo caldera, which produced the world's most recent supervolcanic eruption 27,000 years ago.
Is it likely to erupt again?
In the near future, not likely. Volcanologists say it is possible the next 50 years could bring worrying but benign signs of unrest, but any activity would probably be signalled months, if not years, in advance.
