The sobering story behind why a dog died helplessly in a hot car in Perth this week.

On Wednesday, Perth woman Amy Middleton witnessed a sight she says she would not wish upon her worst enemy: the death of a helpless dog.

She was among those who had attempted to save the Bullmastiff, which had been left trapped inside a Holden Astra for nearly two hours in the suburb of Malga, 11km north of the CBD.

A passerby had managed to free the animal from the vehicle, and a vet nurse attempted CPR. But he couldn’t be saved.

“Today we experienced one of the most gut wrenching situations that not even my worst enemy should have to go through,” Middleton wrote on Facebook. “We watched an innocent animal lose its life to no other but a human being’s fault.

“The human that, that dog would have loved the most in this world. Their owner!”

Yesterday, that owner spoke.

The unidentified Perth man told media he had been living in his car, rendered homeless after losing his son to cystic fibrosis and being without work.

He said was attempting to secure new accommodation when he left Bully in the vehicle, and explained was unable to lower the windows due to the vehicle having a flat battery.

"He was very loyal to me," the man said. "I didn't think it was real, I just thought to myself 'What else, what more? How much more can I take?'

"If I could take it all back I would."

The RPSCA WA said it is investigating the death of a male Mastiff dog, one of two cases of dogs being left in hot cars in Perth on Wednesday.

While the city had reached a relatively mild 27.1 C, the organisation stressed that the temperature inside a parked car can quickly rise to double the outside temperature, and that dog can suffer heat stress and die in as little as six minutes.

"I’m deeply saddened and frustrated by the loss of an innocent dog yesterday, who died in horrific circumstances as a result of people’s negligence," RSPCA WA Chief Inspector Amanda Swift said in a statement.

“Dogs dying from being left in parked cars is not acceptable – it’s an offence; and it’s completely avoidable. There really is no excuse for this to be happening.