Crewman Colin Larsen said the chopper had just dropped off a team of searchers at the top of the track and decided to fly along the Orongorongo River to see if they could see her.
"And on our way back down we basically saw her standing by the river waving her arms."
Rain had fallen through the morning and Ms O'Brien was cold and tired but "obviously very relieved to see us".
The chopper landed about 50m away from Ms O'Brien and she was able to walk over to them.
She was in checked over by a paramedic, but was in good health and suffering no side effects from her night in the bush.
"After spending a night in the forest she was looking very, very well indeed," Mr Larsen said.
"It's always a good day in the office when we go out looking for somebody and you actually manage to find them."
After being reunited with her family, Ms O'Brien told reporters she took a wrong turn on the run.
"I'm not a very good orienteering woman."
She said she was very cold and wet overnight and often thought she would not make it.
"I definitely thought I was going to die."
However, to keep warm, she dug a hole in the ground.
The young mother also drank some breast milk to keep her going. What got her through were thoughts of her children, she said.
"Nothing else mattered but my family."
Much of her time was used to pray, she said.
"I felt God with me the whole time."
The ordeal has not put her off track running, she said.
Her husband, Daniel O'Brien said he never gave up hope of her returning.
Ms O'Brien's father, Andrew Khoo, said he was very happy to have his daughter return.
"Thank you everyone for helping. I am very happy to see my daughter come back."
Ms O'Brien's mother, Maggie Khoo, could not stop tears from flowing and hugged her daughter when she arrived.
A search and rescue operation was launched yesterday when the runner failed to appear out of the hills.
At 7am this morning, the Westpac Rescue Helicopter began searching the area with heat detection equipment in an attempt to locate her.
Additional Land, Search and Rescue teams from the Horowhenua and Wairarapa were also in the area and began searching about 8am.
Breast milk a 'perfect food'
Drinking her own breast milk was one of the best things Ms O'Brien could have done to keep her energy up, a breast feeding experts says.
"It is a perfect food, so you couldn't get better - it's got everything in it," Trish Ebery from New Zealand Lactation Consultants Association said.
"If you've got access to breast milk, you've got access to life."
The nutritious food had "everything" in it, Ms Ebery said.
"You've got brain growers and things for immunity and nutrients to fight infection. It really is the be-all and end-all.
"So wow, what a smart cookie."
Ms O'Brien's milk could have lasted quite some time, even if she was not getting good nutrition elsewhere, Ms Ebery said.
"People in concentration camps and women in very stressful environments, they're still making milk even though nutrition is very, very poor.
"So you could keep going, nobody could say for how long."
By drinking the milk, Ms O'Brien would have been staving off mastitis, giving herself some fluid and nutrition and keeping her supply going for her baby, she said.
