But they were unable to make a belay when the ice turned to hard snow nearer the peak.
Dr Lachmann was about 10m ahead of Ms Hellenbrandt when he heard something behind him.
"I looked back and saw Sabine slipping away ... and then she disappeared into the crevasse," he said.
"I had so much fear that she was dead because I had seen from the side that this was a really deep crevasse, and I could not believe that she could survive such a deep fall."
Dr Lachmann went to the edge of the crevasse and looked down to see her lying on a ledge.
"I shouted to her 'Sabine!' and she answered to me." He said he was glad to see she was alive and moving.
Dr Lachmann climbed down with a rope to help her.
He said her left arm was bloody and injured, and he also feared she might have suffered internal injuries.
"I was really worried she may die on the glacier."
Ms Hellenbrandt said her broken arm and ankle were painful, but after realising she could still move the rest of her body she thought it was not so bad.
Dr Lachmann tried pull her out of the crevasse with a rope, but when that failed he decided to trek to the Pioneer Hut to radio for help.
Ms Hellenbrandt was left lying in her bivouac sack for four hours until she heard a helicopter.
She cried out for help when she heard the three guides who were sent to rescue her.
Ms Hellenbrandt said she felt "very safe" when they reached her.
One of the guides kept close to her overnight while they waited to be airlifted out the next morning.
"I said, 'Please can you come very close to me, then I have the feeling I'm not alone.' And we talked the whole night, a lot of hours, about everything - about mountaineering, about the stars, about families."
She was airlifted to the Franz Josef medical centre the next day.
Ms Hellenbrandt has done a lot of mountaineering in Germany and Austria, but this was her first time in New Zealand.
She said she wanted to return despite her ordeal - but she might not try to tackle the glacier again.
