Dogs could not perspire like humans to cool down - they panted to cool themselves down.
If they were unable to cool down, they slipped into a semi-unconscious state before dying a painful death.
It could take up to two weeks for the animal to die from over heating.
"I've seen dogs die and it's a horrible death. It's just awful," said Ms Martin.
The SPCA said owners needed to leave their animals at home when they went shopping because keeping windows down didn't keep the inside of a car cool enough.
A heat test inside a car with tinted windows at Central Mall earlier this year showed within 10 minutes the heat inside the car rose from 18.2C to 46.6C.
Ms Martin said owners of toy-sized dogs and pitbull breeds were the worse offenders.
Toy breed owners seemed to take their pets everywhere and pitbull owners were afraid of their animals being stolen - those were the main excuses from offenders.
"Just leave them at home, tied up in the shade with a bowl of water," she said.
"Don't take them to town and leave them in the car. It's just too dangerous."
If anyone saw a distressed animal in a car, they should not approach it as it would become more distressed.
Ms Martin said heat-stressed dogs should be doused with cold water immediately.
Meanwhile, if you do come across a dog locked inside a parked car call the SPCA, police or the Rotorua District Council's Animal Control.
Anyone leaving a dog in a car can be jailed up to three years or fined up to $75,000.