UPDATED 3.26pm: The mother and daughter lost for four days in the Central North Island bush have been found safe.

Americans Carolyn and Rachel Lloyd began walking Kapakapanui Track, in the Tararua Forest Park near Waikanae on Tuesday.

A search and rescue operation began yesterday after their vehicle was discovered at the entrance of the track.

Sergeant Anthony Harmer told reporters that the pair were spotted by a helicopter earlier today.

They have been airlifted to hospital for precautionary checks after their miracle survival.

"They planned to only go out for one day over the course of that day. They've ended up becoming a bit disorientated and heading off the main track down into an unexpected waterway where they've become stranded by their circumstances and ended up having to ration their resources [and] stay together for warmth," he said.

"They've ended up in a very steep and rugged area of terrain, so steep at times that they felt that they weren't able to return ... the way that they had come so ended up going further down and ended up in a water catchment area where they were unable to progress out."

The water catchment was a stream of sorts, he said. 

Both were thought to be in good health following their ordeal but were being checked over by medical staff as a precaution.

"They're obviously a little bit worse for wear -- dehydrated and exhausted."

A search and rescue operation began yesterday after their vehicle was discovered at the entrance of the track.

Sergeant Harmer said the mother and daughter were experienced day hikers, having completed the Appalachian Trail and numerous other hikes in their native United States.

He praised them for how they had handled their situation.

"They've been caught out unawares in their circumstances to know fault of their own. It's a New Zealand environment as opposed to other environments that they're been to.

 "They've done all the right things once they have found that they're disorientated - they've stayed together they've conserved their heat, they've conserved their resources so they gave themselves the best chance possible.

"This is a great result for the team and we are grateful for the support of the local community and volunteers who came from Manawatu, Wairarapa, Horowhenua and Wellington to assist in the search”.

The brother and uncle of the two women posted on Facebook to express his relief.

Bob Schumacher said "our prayers have been answered".

"My sister and niece have been found alive. They are being helicoptered out. My sister is okay, my niece is a little weak, but both should be just fine."

Miss Lloyd had travelled to New Zealand to study at Massey University for a semester. Her mother is visiting from their home in the United States.

Massey University spokesman James Gardiner said Rachel was at the university on a one-semester exchange and had arrived from the US in February.

"We will offer Rachel's father transport to New Zealand, and are offering the family our support," Mr Gardiner said.

"We don't know what the situation is yet, but it is very worrying."

Tykori K. Saunders posted on Facebook that one of his "best friends" was missing.

"My heart goes out to you and your mother Rachel Lloyd. I pray that you and you're found safe and sound soon! I love and miss you Rachel." [sic]

Mackenzie Hines asked her followers to "say a prayer that this sweet sorority sister of mine, Rachel, and her mom are found and are okay".

Tessa Bradlie Souffrant posted "My grandbig and her mom are missing while she is abroad in New Zealand".

"Please pray that they find them both alive and well!"

Tararua Tramping Club member Stuart Hutson said the biggest concern for the pair would've been controlling their emotions and keeping warm.

"It's a pretty easy track but it would depend on their experience.

"Novices could become quite confused, they should've taken a map or compass with them, or even a GPS smartphone app."

Tararua Tramping Club president Peter Barber said it would be difficult to get lost on the track.

"You go up one side and come down the other.

"But it is quite steep. The weather's been pretty good. It can be harder when it's not so good."

Mr Barber said it was important hikers always told someone where they were going, and carried good gear, extra food, a personal locator beacon and GPS.