A Westpac Rescue Helicopter spokesman said Mr Carter went into cardiac arrest shortly after they had arrived to treat Mr Hopkins. He said the event had not been running long when the two men went into cardiac arrest, just before lunchtime.
The two incidents were "moments apart", he said.
Today, Mr Hopkins' family paid tribute to the 6ft-3in engineer who was a "great outdoors fanatic."
His ex-wife Sandy Galland said: "Hoppy was a really fit guy and didn't have any health problems so this came as a real shock to us all.
"He was a great outdoors fanatic - a huntsman, cyclist, runner, member of the Hash House Harriers, and a very experienced endurance racer, going right back to the very first Speights Coast to Coast. He was good mates with Robin Judkins and was involved in that from the very beginning.
"It's been a huge shock for us all. Nothing has really set in yet."
Ms Galland said fellow racers tried to resuscitate Mr Hopkins on the trail before emergency services arrived.
She said: "They gave him CPR until professionals got to him.
"He was larger than life, called a spade a spade, and would do anything for you. He was very passionate about sports, the outdoors, and teaching kids.
"He lived a very healthy lifestyle, but loved a party also.
"His children and his sports were his life. We're expecting a big funeral on Friday."
He leaves behind two daughters - Zara, 10, and 12-year-old Dawn, who said her father "taught us everything he knew."
Dawn Hopkins said: "On Saturday morning, we were having fun jet-skiing and playing in the water with friends and then we got a phone call about dad and our day went from great to terrible.
"Dad took us hunting and showed us how to do everything. He taught us really well.
"He always pushed us to do sports and to get right up there, and I will continue to play my sports because that is what he would have wanted.
"He will be missed heaps."
Race director Rod Hibberd of Blue Dog Events said the death will affect the Christchurch mountain biking community, which he described as being "very tight knit."
He said: "Riders all know each other and will be greatly saddened at the loss of a fellow rider. Like myself, I know everyone will be thinking of the family at this time."
