Dog owners are being warned that leaving their pets in cars this summer could leave the animals brain damaged or even dead.

The SPCA said so far this summer 10 dogs had died or suffered severe and debilitating heat stress through being left in cars.

"Our summer temperatures can be merciless, and overheating causes appalling suffering and can lead to an equally appalling death," national chief inspector Charles Cadwallader said.

"There are many reasons why you might take your dog in the car with you - you may think that you're doing him a favour, giving him company. But the moment you step out of that car, leaving your pet at the mercy of the elements, you risk losing more than just your best mate."

Even with the windows slightly open, a car parked in the shade on a 30degC day will heat up to 39degC in 10 minutes. In 30 minutes, the temperature will hit 49degC, and on hotter days it will go even higher.

Locked in a hot car, a dog can only withstand temperatures of over 40degC for a very short amount of time before suffering irreparable brain damage - or even death.

Dogs also need access to plenty of water and cool, fresh air in order to fully moderate their body temperature, Mr Cadwallader said.

The SPCA will prosecute and offenders could be jailed for up to three years, or may face a fine not exceeding $75,000, or both.

The New Zealand Veterinary Association said many incidents go unreported.

"Leaving your pet inside a car is similar to placing them inside an oven - essentially you're leaving them to cook", president of the Companion Animal Society branch of the New Zealand Veterinary Association, Pieter Verhoek, said.

Dogs do not sweat like humans, they pant to dissipate heat and cool their body temperature.

"It can take less than 20 minutes for a dog to overheat and slip into a coma. Frantic panting is the most common symptom, along with wide-open eyes and a red tongue.

"Even if the dog recovers there can still be permanent internal damage," Mr Verhoek said.

Anyone who sees a dog in distress locked in a hot car should call the SPCA or phone 111.

First aid for a dog suffering from heat exhaustion involves immersing it in cold water until its body temperature is lowered.