Most people love their pets, but they can make bad decisions about their care, said SPCA chief executive Ric Odom.
De-sexing animals, especially cats, is a necessity for owners, said Mr Odom.
"It's the responsible thing to do. We have such a problem in New Zealand with unwanted animals."
Mr Odom's said the advice about dogs and cars was simple: "Do not leave your animal in the car on a summer's day."
He said people may only intend to be away from their car for a short time but there was every chance of being held.
"Then they come back to find their pet severely distressed or worse."
All animals - especially dogs - needed a lot of water and could easily overheat.
Mr Odom said the majority of New Zealanders were responsible owners and some just needed a bit more education.
"And some of those that do not are not bad people they just do not think," he said.
NZME