A new RSPCA report titled `Dogs Die In Hot Cars' says nearly 500 complaints about animals in cars were received over the past five years.
And it seems Sydney residents are the worst in the state for leaving their pets in dangerously hot cars, the RSPCA says.
Of 466 complaints, 94 per cent related to dogs and 21 occurred in the City of Sydney council area.
RSPCA staff were forced to rescue 17 animals during that time and around six died.
The report released Tuesday also found cars parked in the sun could reach temperatures of up to 75C and remained hot even with the window slightly open.
RSPCA NSW CEO Steve Coleman said he didn't think people realised how easy it was for animals to suffer heat stress.
"Just six minutes is all it takes for animals to succumb to a pretty tragic and stressful death," he said.
"People wanting to do the right thing and take their animals out for the day... sometimes tragically forget that they've left their dogs in the car."
He also said that metal ute trays "can become... hot plates" for animals.
Owners face fines of up to $5500 and a six-month prison sentence if their animal suffers as a result of being left in a car.
A maximum penalty of up to $22,000 and up to two years' jail apply if the animal dies.
"We've prosecuted many people and we don't take it lightly," Mr Coleman said.
The RSPCA has joined with the City of Sydney to launch a public awareness campaign on the issue.
They're hoping 20,000 people will make the pledge to never leave their dogs in cars at justsixminutes.com.au.