Mr Mitchell, from the North Shore, said he blacked out during the fall but knew he was seriously injured as soon as he came round.
"I knew it was in the pelvic region, I could feel the bones moving around, so I thought yep, it's broken. I wouldn't wish it on my worst enemy."
Yelling desperately for his girlfriend to call 111, Mr Mitchell was cut about the body as the waves smashed him against the rocks.
"I just let the water take me, there was nothing else I could do. I was in the water and I had given up."
Then the man appeared.
"Next thing I know he hauls me on to the shoreline and he's saying, 'I'm here to help, don't worry mate, my name's Jason, I'm an ex-Marine'."
Jason, who said he was aged 52, protected Mr Mitchell from further injury by holding his body out of the water for half an hour.
"I count my lucky stars that he was there," said the supermarket worker.
But when emergency services arrived, his rescuer disappeared.
"I just want to thank him from the bottom of my heart for what he did and just let him know that I'm extremely grateful," said Mr Mitchell.
Trying to get access to the blowhole was "extremely challenging", said St John Ambulance operations team manager Patsy Carlyle.
The Fire Service's high angle rescue team was called in from Auckland and he was airlifted to hospital by the Westpac rescue helicopter.
Doctors told Mr Mitchell he had a chipped vertebra and his pelvis was broken in three places. He had surgery to his pelvis, and will be in North Shore Hospital for six weeks.
