The skipper of a boat hit by a rogue wave has told a coronial inquest he knew his son was dead when he became blue and unresponsive after spending hours in Foveaux Strait.
Barry Bethune was at the helm of the catamaran when it flipped and sank 30 minutes into a fishing trip in January this year.
Mr Bethune's 23-year-old son Shaun Bethune and his best friend Lindsay Cullen, 59, died after the sinking.
Sisters Denise Zonneveld and Carol Saxton and Mr Bethune survived the ordeal after being rescued off Ruapuke Island late on January 3.
All five on the 7.25-metre Extreme 1 were wearing lifejackets.
The wave that forced the boat to capsize was unexpected, Mr Bethune said.
"I didn't see it coming at all," he told Tuesday's inquest.
All five made it onto the upturned hull, but wet cellphones meant they could not raise the alarm and an emergency locator beacon was left on the boat which sank after 10 minutes.
Mr Bethune told the inquest he became aware his son and best mate were dead after the group spent hours huddled together in the water, the Southland Times reports.
"His (Shaun's) eyes were closed and he was blue ... I knew at that time he was dead ... I looked over at Lindsay. He was on his back and he had fluid in his mouth as well. The fluid was sitting there in his mouth, not moving. He was blue. I knew he was dead as well," Mr Bethune said.
The pair were let go and their bodies were recovered the next day.
The survivors were rescued by Easy Rider skipper Rewai Karetai who rowed out to save them after hearing their cries for help. Mr Karetai and seven others died in the same stretch of water two months later when the Easy Rider sank.
Coroner David Crerar ruled the dead men, clad only in jeans and cotton t-shirts, had succumbed to hypothermia and urged boaties to carry back-up communications equipment.