The group went out on Mr Bethune's twin-hulled boat, an ex-commercial paua vessel, so Mr Cullen's friend and her sister visiting from Kapiti could go fishing.
"But it just turned to tragedy."
The boat was doing about 15 knots [28km/h] when a wave struck at an angle and flipped it about 6.30pm.
All on board were thrown across the cabin and Mr Cullen suffered a blow to the head.
Seconds later, the cabin filled with water and the group had to scramble out.
Mr Bethune helped one of the women out, then tried to get back into the boat to reach an emergency locator beacon.
"I tried twice to dive down in through the window, but twice I got hit by big waves. I was already banged up, and I just couldn't get in there."
The group perched on top of the boat, holding on to a rope they tied to one of the hydrofoils.
They tried to use their cellphones but they were too wet.
Waves kept pounding against the boat as it drifted to about 1km off White Island, where it started to sink.
"Within the space of about two minutes it went from being flat to sinking beneath us, so we all had to watch out that we didn't get caught up in the ropes as it went down."
The group got into a huddle to keep warm, with Mr Cullen between the two women because he "wasn't very flash" after suffering a possible concussion.
Mr Bethune and Shaun, the two strongest swimmers, were on opposite ends of the huddle as they swam towards White Island.
"We thought we were going to make it, but then the current ... we ended up drifting past it. We probably missed it by about 200m."
After a quick discussion the group headed for Ruapuke Island.
By about 10pm - half an hour before nightfall - the current had taken them past Caroline Bay.
"We had a renewed effort to try to get towards where it was, we made really good ground," Mr Bethune said.
"Then I turned around and Shaun was unconscious, and I checked to see if he was alive. But he was dead.
"And then my best friend Lindsay, I checked him and he was the same. They both died of hypothermia. They'd just gone unconscious, they didn't drown.
"I wasn't sure if we were going to be able to make it or not, so I made the decision to leave them. They had lifejackets on so I knew they'd float."
The remaining three later spotted a boat about 100m away.
"So I swam by myself, tried climbing into the boat but it was too rock-and-rolly for me to get into it. So I started yelling, 'Come and help us, please help us'. Some people on shore heard us."
A man came out on a boat, but Mr Bethune said he couldn't remember exactly what happened next.
"I can remember somebody helping me walk, I couldn't walk by myself - I was just too unsteady up the beach."
Police said the three survivors were rescued by the owner of the fishing vessel Easy Rider, who helped carry them back to his camp.
A rescue helicopter arrived about 10 minutes later and took the group to Southland Hospital, where they were assessed for hypothermia and allowed to go home.
Mr Bethune said none of them would have survived were it not for his son's efforts.
"If he hadn't gone on that other side and paddled and swam, he could've made it by himself. But he chose to help those other people."
He described his son - a wiry 1.7m dairy farm worker who friends knew as Shorty - as a "rough diamond".
"Just in the last six months he'd started a new job, he was going really well. He was looking forward to keeping his head down and becoming a farm manager, and he would've. He had the skills - he's been farming long enough."
Shaun left his partner Sharyn and two young sons aged 2 and 6 months. "She's lost her soulmate. It's just devastating for his little family," Mr Bethune said.
Shaun was also close to his uncle, Pete Bethune, who last night described his nephew as a "tough little bastard" .
"But it doesn't matter how tough you are - if you haven't got much fat on you (the cold water) does clean you up. And I think that's what has happened to Shaun."
Pete Bethune said his nephew had crewed with him on the Earthrace vessel in the south of the South Island, and had spent a couple of months with him and Sharyn travelling around Australia.
Barry Bethune said he had been asking himself if he could have done anything differently.
"I just wish I'd never gone. I've lost my son, my hunting mate, my diving buddy, my fishing buddy and my best mate."
He stressed the importance of always wearing life jackets, and said skippers should always keep their emergency beacons on them rather than in their boat.
Police said both bodies had been recovered, one from Ruapuke Island and the other from the sea nearby.
They were investigating with assistance from Maritime New Zealand, and the deaths had been referred to the coroner.
- additional reporting, Jarrod Booker
