One dog has died and two others are recovering from potentially dangerous heat-stroke after being left in parked cars in Perth.

Passers-by spotted a large mastiff in a car parked in Balga in Perth's north around 2:00pm on Wednesday, but by the time they managed to pull it out it was suffering seizures.

They performed CPR but were unable to save the dog.

At the same time, Emma Stanes came across two distressed dogs in a parked car near Belmont Forum in Perth's south and looked for the owner in nearby businesses but found no one.

She said after nearly 45 minutes she became concerned for the dogs, which were panting and visibly distressed, so she contacted the RSPCA who alerted police.

"If it came down to it I was going to try open the car or smash a window," she said.

A large dog was locked in a car in Perth and later freed after its owner was confronted. ( Supplied: Emma Stanes )

The owner of the dogs then returned and allegedly became hostile, so Ms Stanes began filming him.

"What got me was that you could tell that he really loved his dogs, but was just ignorant to the fact that dogs can die in a car," she said.

"He kept saying 'they're alright it's not that hot'.

"But anything could have happened, 45 minutes in the heat — it can happen quicker than that. They might have died."

The dogs were given water by the owner, who left with the dogs after speaking to police.

The RSPCA is investigating both incidents and the owners could face fines of up to $50,000 and five years in prison if found guilty of animal cruelty.

A white dog locked inside a car by its owner was later freed.

Even cool weather poses risks

The dog owner spoke to police before leaving the scene. ( Supplied: Emma Stanes )

RSPCA Chief Inspector Amanda Swift said there was no safe time or way to leave animals in a parked car regardless of weather conditions.

She said even in cool weather with windows down and water readily available, it can take as little as six minutes for an animal to overheat in a parked car.

"Obviously a dog is covered in a fur coat — you put it in a hot box and it turns into an oven," she said.

"You think because there is a wind or cloud cover the animal is going to be OK.

"But it isn't, the car heats up, they can't pant, they can't sweat in the same way we can, they overheat."

Ms Swift said the two dogs who were rescued may not be out of the woods yet.

"It's not necessarily just at the time. You may think you've got away with it, but dogs die two and three days later," she said.

"The blood coagulates. It basically means the blood thickens and the animal dies."

More dogs dying as people ignore pleas

RSPCA Chief Inspector Amanda Swift said owners needed to realise how quickly cars could overheat. ( ABC News: James Carmody )

Ms Swift said dogs and other animals left in hot cars was an issue RSPCA inspectors dealt with on a daily basis.

She said instances were becoming increasingly common, with 132 reports of dogs locked in cars so far this summer.

"Even now as I'm leaving there's two dogs in a car again, and we all know how hot it is today," she said.

"What more can we say to people other than to stop doing it, stop being stupid.

"Stop putting your animals at risk. This is neglect. It is cruelty and you can face prosecution."

Ms Swift commended the efforts of the members of the public who intervened in Wednesday's incidents.

"The RSPCA and the police and the rangers just can't be everywhere and this is happening all up and down our state," she said.

"Members of the public are stepping in and we're seeing they're facing aggression and abuse as well from the owners who come back and they're putting themselves at risk for other people's animals.

"Don't risk your animals and don't abuse other people who are only concerned for your animals."