Heat stroke in dogs can kill a dog in a very short space of time. Newfoundland dogs are particularly prone to this condition because of their heavy coats and their breeding, which is designed for cold climates.
Newfoundlands very often die very quickly when left in cars, and it doesn't even need to be particularly warm weather for the dogs to suffer.
I never leave my dogs in my car for more than a couple of minutes, no matter what the weather.
When my Joshua was only ten weeks old, he developed heat stroke. He was in the house with Ferdie, and it was a particularly hot day.
First he began vomiting, then staggering about as though he could not get his balance. At only ten weeks, I was naturally quite frantic, but since he had received his booster vaccination the day before, I believed it was a reaction to that.
I rushed him to the vet, who took his temperature and diagnosed heat stroke. This little pup spent the next five hours sitting in a paddling pool at the vet's surgery, with a drip in his arm.
This horrible experience taught me one important lesson. Never wait and see how things will progress. Had I waited till the following day, I would have lost my lovely little puppy.
Always consult a vet straight away.
More detailed information about heat stroke in dogs can be found at Heat Exhaustion in Dogs.