The incident occurred around midday when the outside temperature peaked at 25 degrees and there was no wind.
Ms Keys said the baby looked extremely uncomfortable so she slid open the back door unclipped the seatbelt from his car seat and gently pulled him out.
"As soon as he was out of the car and in my arms, he stopped crying."
Police say that young children must never be left alone in a house or vehicle and they require constant supervision.
It is illegal to leave a child under the age of 14 without reasonable provision for their care.
Ms Keys said she couldn't believe with all the publicity about the danger of leaving a child in a hot car that it still happens.
"Heatstroke can happen in two minutes. Brain damage can occur. Death is then not far away."
In 2015 a 16-month-old baby died after being left in a car outside of the mothers Whanganui workplace.
Ms Keys said she approached the man and told him that the baby was very hot and distressed and about the dangers of leaving a baby in a hot car.
"He protested that he'd 'only been gone two minutes' and that 'he was asleep' when he parked the car."
She said the man didn't seem that concerned and told her that the baby usually cries and it was normal.
"I said to him that most babies need to be held a lot, they need to feel secure, and being held and soothed helps them to have that sense of security."
But Ms Keys said even though the man agreed not to do it again, she was unsure he understood how dangerous it was.
"If you are at the beach and you see a child distressed in the water you go to save them, the same principle should apply to a baby left in a car."
