"To be honest they had all the equipment - they had life jackets, helmets, everything and if they'd been a half an hour or hour earlier they would have got through before the flood came.
"If they were half an hour later they wouldn't have launched the boat because it came down as a wave of water.''
The 64-year-old farmer, who is an experienced jet boater, said he and his son Andrew launched the boat from Kuratawhiti street, near the Waiohine Bridge.
The pair picked up Hopkins' wife about 12km along the river.
"She was in a state of shock.''
Mr Slater said when she was rescued, the second man had managed to escape the river but Hopkins had already been picked up by the Westpac Rescue Helicopter.
"There had been 89mm of rain in the ranges and the river rose from 10 cubic metres per second to 300 cubic meters per second within about 15 minutes.
"It was some big water in the gorge and they were just in trouble really.''
Mr Slater said he lived about 5km away from Hopkins and his wife.
"I didn't know them though.''
He has been involved in previous water rescues and went out on his boat on the Waiohine River at least once a week.
"They were just in the wrong spot at the wrong time.''
Hopkins, along with Ethan Van der Ryn, won an Oscar for their sound editing work on King Kong in 2006.
The pair also won an Oscar for best sound editing on the second Lord of the Rings movie - Lord of the Rings: The Two Towers in 2003.
He was nominated for a Golden Globe for his work on the Transformers movie, but did not win.
Hopkins had a career in sound editing for more than 60 movies and television shows spanning four decades.
He worked on all three Lord of the Rings movies, as well as Kung Fu Panda, and Dreamgirls.
He also worked as sound editor on Sir Peter's 1994 film Heavenly Creatures.
