Wade Briscoe narrowly escaped through the window of a sinking vehicle but his long-time friend Ian Horrax did not.
Mr Briscoe (75) yesterday said Mr Horrax (68) appeared to have a medical event before his Toyota Rav 4 plummeted into the Turnbull River.
He had been driving across farmland about 7.30pm on Monday, about 15km south of Haast, when the vehicle veered through scrub, just missed a large tree, and launched at speed into the river from a 6m-high bank.
''It was totally uncharacteristic of him [Mr Horrax] driving in that manner and/or treating his vehicle in that manner,'' Mr Briscoe said.
He believed the route strongly indicated Mr Horrax died from a medical event before the vehicle plunged into the river.
As the Rav 4 began to float downstream, Mr Briscoe found the car electrics were still working, so he wound down his window and called to friends nearby for help.
''Then the car started to sink and just volumes of green water came in and it was very hard to get out ... but I did and swam to the bank where one of the guys had a rope ... I just thank God that I'm here.''
Mr Briscoe, who lives in Wanaka, had known Mr Horrax since 1975 when both men lived in Invercargill. Nearly 20 years ago, Mr Horrax moved to Albert Town, where he and his wife Ann ran bed and breakfast accommodation.
Mr Horrax was widely-known and respected for his fly-fishing skills and did a lot of guiding with mainly overseas clients, Mr Briscoe said.
Both men belonged to a syndicate which owned two whitebait stands and a hut near Haast and had been visiting the West Coast for years, where they enjoyed ''many rich and happy experiences''.
Mr Briscoe praised the support on the night from the district nurse and the actions of Haast policeman Senior Constable Robin Manera, who twice dived into the river to the sunken vehicle.
Snr Const Manera said the car washed 100m downstream where it lodged behind a log on the bottom of the river.
It was retrieved later that evening with Mr Horrax's body still inside.
Police are investigating the cause of death and would await the result of a postmortem to determine if Mr Horrax had suffered a medical event.
