"When the guys found him on the ledge Ben thought he was having a heart attack but the paramedics said he wasn't having a heart attack - he was dying," she said.
"He would have been gone in a few more hours, he wouldn't have survived another night and he would have shut down."
The father of one, who was found about 1.3km from his father's home, about 20km south of Taupo, had gone in to the Lake Taupo forest to cut a track for his horse.
His partner dropped him off at Te Oranga Rd about 10am on Wednesday but called police when she returned home later that night and he wasn't there.
Mr Severne had slipped down a waterfall on the Waitotara Stream, knocking himself out and losing his GPS, torch and balaclava during the fall.
While not seriously injured, he was stuck on a ledge that was inescapable on either side or above and with a 50 metre drop below.
Dressed only in trousers and a thin shirt, he readied himself for a long night while his dog, Whai, a German short-haired pointer, howled from above.
Ms Severne said her brother told Whai to go and get help but she wouldn't leave him and instead jumped from above into his arms.
He tried to sleep during the day and stayed awake at night on the ledge, which she said was no bigger than a family car.
By Friday dozens of whanau, including some who flew back from Australia, a search and rescue team and friends within the local hunting community had rallied to look for her brother.
But while Mr Severne could see a plane and helicopter in the air he couldn't hear the yells of those looking for him as the noise from the waterfall made hearing anything almost impossible.
With no food and surviving only on water from the stream, Mr Severne, who is also on heart medication, lasted three long nights but his sister said by Saturday morning he was almost certain he couldn't go on.
"He was cold the whole time, but on Saturday morning he couldn't stop shaking- he tried to keep warm but he realised by then he couldn't have done another night, it was freezing and he wouldn't have made it."
One of Mr Severne's friends, Brett Cooper, whom he helped to find in similar circumstances in the bush about 15 years ago, found him after finding a padded area of bush nearby and a piece of Mr Severne's clothing.
The man then saw Whai on the ledge and his mate's legs sticking out.
"His mate told Ben, "I thought I owed you for saving my life so here I am"."
Mr Severne spent Saturday night in Taupo Hospital recovering from hypothermia with his daughter. He returned home today.
Senior constable Barry Shepherd of Taupo police said Mr Severne's survival story was a remarkable one but searchers hadn't given up hope of finding him alive.
"It's a remarkable survival story but people can do amazing things in that kind of adversity," he said.
- Additional reporting APNZ
