"I went to go open my eyes but all I could see was little white bubbles so I just closed my eyes and tried not to use my energy because then I use up my breath.
"I got to my last breath. I couldn't really think at the time."
Gavin popped off his weight belt and floated to the surface before realising his arm was "dead" and he was unable to swim.
"My cousin was about 30m from me and I could hear his flippers from a mile away trying to get out to me because he saw me pop up.
"As soon as he got there, he jumped behind me and lay behind me, held me and kept kicking his flippers to keep buoyant. So that's when I got my time to rest and just relax and breathe."
Gavin said they saw a float pop up and then go straight back down again as the orca dragged it away.
The outgoing tide then carried Gavin to the safety of some rocks, where he waited for feeling to return to his arm.
His gear was never retrieved.
A day later, he was taken to Whangarei Hospital, where he stayed briefly while doctors determined that he did not have water in his lungs.
Just before the attack, Gavin and his cousin had started to make their way to shore after seeing a pod of orcas 300m away from where they were diving. But the whale had swooped in too fast for them to evade it.
Gavin said: "Everything was going wrong that day, everything was meant to stop us from going out.
"My cousin didn't want to go out ... He offered to give me a knife to strap around my leg and I said no to that. If I had had the knife I could have cut the rope."
There have been more than a dozen deaths and serious injuries from attacks by orcas in captivity, including one depicted in the 2013 documentary Blackfish.
However, experts say attacks by orcas in the wild are rare, and there are no known fatalities.
Whale Rescue co-founder Jo Halliday said orcas were "just like humans" in that they had highly developed brains, were naturally curious about their surroundings and lived in tight social structures.
Fewer than 200 individual killer whales have been recorded as living in the waters around New Zealand.