"But a human so selfish to lock a dog of a type of mastiff breed, a big boy, in a tiny Holden Astra, all windows up and that was completely full to the roof with junk in the warm weather of today!"
She said the group who found the pooch did everything in their power to save it.
According to the post, the dog was carried to a nearby shaded area by a man. A veterinary nurse, who drove by the incident, pulled over and attempted CPR on the animal.
"It just wasn't enough, we lost him and everyone there at that moment was beyond sorry to that animal," Ms Middleton wrote. "We were all utterly heartbroken and angry that anyone could do this!"
Ms Middleton said the incident shows that some pet owners need to wake up to the danger of leaving an animal in their cars.
"They aren't able to let themselves out, instead they suffer and in some cases like this, they lose their lives," she wrote.
"People need to start thinking a lot more when it comes to their animals! A pet loves you unconditionally and wants nothing but the same in return, if you can't give them that then don't own an animal! It's pretty simple.
"Rest easy big boy, you didn't deserve this. I hope you know how much that group of strangers loved you for that short time & will care for a lifetime."
Local pet community Facebook page, Perth Pets in Need, posted about the incident on social media — saying the dog suffered two seizures and threw up before he died a slow painful death.
"Like all dogs who get heatstroke, he suffered horrendously," a spokeswoman for the group wrote in the statement.
"There is absolutely no excuse for locking your dog in any car with no windows down at any time of the year, but particularly not on a humid summer's day.
"Huge thank to those who tried to save this beautiful boy. We are so glad that you were there to bring this poor dog some relief in his final moments on this Earth. Our thoughts are with you and this darling furbaby."
