Scientists have worked out how much rain fell in the Esk Valley catchment near Napier during Cyclone Gabrielle - 72 Olympic swimming pools every minute, for six hours.
It caused the Esk River to almost double its normal level in less than 12 hours.
Cyclone Gabrielle dumped an astonishing amount of water and now we understand why it filled the valley faster than many residents could escape.
"This rainfall greatly exceeded what would normally be expected from historic measurements at this location," NIWA hydrodynamics scientist Dr Graeme Smart said.
Data from Metservice shows how much rain fell in and near the Esk catchment on the day of the catastrophic flood.
The rain started at 1am on February 13 and then steadily increased.
The rainfall station in Glengarry shows it peaked between 10pm and 7am on February 14 when 34.6cm of rain fell. The Maunganui rain station at the same time recorded 21.3cm and the Te Pohue station recorded 24.9cm.
The effect on the Esk River of that amount of rain, both falling and flowing down from the valley, was rapid and staggering.
The river's normal level is 9.6 metres but the monitoring site at Berry Road shows that by 10:05pm on February 13, the river level was 11.2 metres. By 4:20am, it was at 18 metres.
The river level almost doubled in less than 12 hours as a phenomenal amount of water fell in the catchment of 252 square kilometres.
"This is equivalent to about 72 Olympic swimming pools of water every minute over a duration of six hours. It's an enormous amount of water coming down," Dr Smart said.
Hawke's Bay Regional Council interim chief executive Pieri Munro is on record saying rainfall in the Esk Valley catchment exceeded all expectations and they expected it to be heaviest elsewhere.
But MetService points out it issued a red warning for the entire region - that means expect a state of emergency and take action immediately.
History reminds us that Hawke's Bay residents have been here before. The news reports from Cyclone Bola were strikingly familiar.
But MetService said a warmer atmosphere due to climate change, as well as marine heatwaves, means today's storms are more violent.
"This energy that's in the atmosphere feeds into these tropical cyclones and cyclones as they come down over New Zealand, and therefore they do pack more of a punch, they will have stronger winds, they will have more rainfall," MetService head of communications Lisa Murray said.
In fact, NIWA said in Esk Valley, a 250 square metre house would have had 65 tonnes of rain fall on it in just six hours.
While Esk Valley recovers, the question looms - when will we learn that we must be better prepared? Because history will repeat itself.
