River level data from the Waiohine Gorge has revealed the size of the wall of water which caused the death of Greytown man Michael Hopkins in a rafting accident on Sunday.
Greater Wellington Regional Council's river gauge, 3 kilometres upstream from where the Waiohine emerges from the gorge, shows a massive spike in the river height at 4pm.
The water level rises suddenly from about 30cm to peak at a height of almost 2.2m.
River flow increased from about 20 cubic metres per second to 300 cubic metres per second at the same time.
The wall of water had receded to less than a metre high by midnight.
Rainfall levels at the Waiohine River at Carkeet, in the Tararua Range at the head of the Waiohine catchment, shows almost 50mm of rain fell on Sunday, with a peak of 15mm per hour between 1pm and 2pm.
Mr Hopkins, an Oscar-winning sound editor, was killed while rafting on Sunday afternoon with his wife and another man.
The trio were in a 4m inflatable raft.
Fred Mason, a relative of Mr Hopkins, who lives in Canada, has queried whether early-warning systems could be put in place on the river.
Mr Mason said his wife was a cousin of Mr Hopkins.
"We are all heartbroken over Mike's death; he was a passionate and splendid human being who was a true friend and a real anchor for his extended family," Mr Mason said.
The increase in water level shown by the river gauge represented an "absolutely phenomenal increase in flow, over a very short period of time".
"I don't know if there have been many injuries or fatalities in your region in the mountain gorges and rivers over the years, but if this stream flow is not extraordinary, then I think that it would be a useful to think that whatever body provides hazard warnings would extend their services to perhaps preclude another tragedy."
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