He does not remember the moment his heart stopped - or the hours before and the four days after. He now faces an eight-week recovery from his head injury and fractures and will never be able to participate in endurance events again.
"I don't remember any of it," he said. "I remember registering my bike the day before and then having breakfast in the morning and then I woke up on Thursday."
Last Tuesday, Mr Nadollek, a manager at a construction training school, had an implantable cardioverter defibrillator (ICD) inserted into his chest, and is now recovering at home in Blockhouse Bay.
His wife, Sarah, said that on Sunday, January 18 - the first anniversary of the family's migration from Britain - she was making breakfast with their daughters Leah, 12, and Abbi, 10, when she was summoned to Auckland City Hospital.
"By the time we got to the hospital, he was on a ventilator, and they didn't know what was wrong and it was quite scary," she said.
"They took lots of tests but couldn't really find anything and so we still don't know why his heart stopped."
Mrs Nadollek believes timing was everything for her husband. "The right people were there at the right time. Had it been a training Sunday or even in the water earlier, he wouldn't be here.
"One of the doctors who stopped specialised in resus[citation] and she gave him CPR. She's the reason Steven is alive."
Now, if his heartbeat becomes irregular, the ICD emits low-energy electrical pulses to correct the rhythm.
But the lifesaving device means his involvement in endurance events and contact sports is over.
Mr Nadollek was an engineer with the British army for 22 years before moving to NZ, a place the family chose so they could enjoy the outdoors.
He was on England's cross country ski team in his teens and has competed in countless half-marathons, marathons, triathlons and endurance walks. He had already completed one half-ironman as part of his 10 months of training for the New Zealand Ironman next month.
His wife and daughters were also involved in triathlons and sports.
"I can't do anything other than be a support person now," said Mr Nadollek. "My family have supported me in everything, so it's payback."
Ironman's NZ director, Janette Douglas, said it was "extremely rare" to have major accidents at the events, but there was a raft of response strategies to deal with any.
The device
*Implantable cardioverter defibrillator (ICD) implanted into the chest.
*Uses electrical pulses or shocks to help control life-threatening irregular heart beats.
*Wires with electrodes on the ends are connected to heart chambers.
*Heart rhythm is monitored and if it becomes irregular, the ICD uses electrical pulses to restore a normal rhythm.
*Pulses become larger if heart rhythm doesn't restore.
Source: National Heart, Lung and Blood Institute
