Choking on a Crunchie bar is what caused a car to plunge 35 metres in to Waipunga Stream on Tuesday afternoon.

Police were amazed the male driver in his 50s and his mother in her 70s were able to escape the crash on State Highway 5 with minor injuries.

Acting sergeant of the Hawke's Bay road policing unit Grant Marshall said the pair were discharged from Taupo Hospital at 10:30pm on Tuesday.

"The woman was treated for mild hyperthermia but we figured the branches of the tree took a lot of the energy which meant it was luckily not a fatality," Mr Marshall said.

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The man who was driving his mother back to Hawke's Bay braked to pull over after choking on a chocolate bar 10 kilometres north of the Tarawera Tavern and plunged down a vertical cliff.

Mr Marshall said there were no skid marks from the car, so he believed the man aquaplaned.

"There was torrential rain and he was not used to the road nor the car he was driving, as he had borrowed it," Mr Marshall said.

Mr Marshall said the male driver believed he had fallen unconscious after they crashed, as he woke to his mother screaming ''get out of the car''.

Luckily the driver was able to scramble up the vertical bank in bare feet and wave down some cars which pulled over and notified emergency services.

Mr Marshall said police were surprised the man made it up the overhanging cliff, because the bank was so steep. He described it as a vertical drop.

"If he hadn't made it up there it would have been a lot worse. The car was nestled in to the bank by the stream and there were no marks on the road showing traces of the crash, so no one would have seen it."

He said a person must walk to the shoulder of the road to see the car.

The level three abseiling rescue team from the fire service abseiled down and brought the woman back up on a stretcher. She had somehow managed to clamber to the top of the car.