The RSPCA is considering legal action against the owner of a dog which died after being left outside with no water and shade in Whyalla, while flying foxes in Adelaide and Melbourne are suffering in record heat.

Key animals: A dog has died in Whyalla after being left outside without water in the heat

A dog has died in Whyalla after being left outside without water in the heat Flying foxes are falling from trees in Adelaide

Flying foxes are falling from trees in Adelaide December temperature records have broken in many parts of the country

It comes as the Bureau of Meteorology reveals yesterday was Australia's hottest day on record.

The RSPCA said the one-year-old dog's remains had been taken to Adelaide for an autopsy after temperatures reached 42.1 degrees Celsius in Whyalla.

RSPCA chief inspector Andrea Lewis said pets must be given ample shelter and access to clean water during extreme heat.

"Ideally, pets should be indoors with air conditioning on — but at the very least they must be able to shelter from the sun through the day and have plenty to drink," Ms Lewis said.

The RSPCA advises pet owners should also consider offering cool snacks, make sure animals with longer hair are groomed and treatments for fleas and ticks and other vaccinations are up to date to avoid additional discomfort.

Exercise and walking in hot weather, and on hot pavement, should also be avoided.

RSPCA Inspector Cheryl Doudle said some pet owners still were not getting the message, with call outs for dogs in distress.

"We received a call about an hour ago from a distressed member of the public at Glenelg. They'd seen a dog in a car for about half an hour," Ms Doudle said.

"While we were on the phone to them, the person came back and drove off with the animal, so we're very concerned about that situation."

Fauna Rescue of SA volunteer Suzanne Pope with a grey-headed flying fox pup. ( ABC News: Brittany Evins )

Flying foxes dehydrating in heat

In South Australia and Victoria authorities are working hard to assist endangered grey-headed flying foxes, which are known to fall out of trees during extreme heat.

SA Fauna Rescue volunteer Suzanne Pope said the flying foxes in Botanic Park just outside of the CBD were very low in the trees because they were in distress due to dehydration and overheating.

A grey-headed flying fox pup suffering in the heat in Adelaide's Botanic Park. ( ABC News: Brittany Evins )

She said pups were especially vulnerable and one had fallen "straight out of the sky".

"We give them fluids, keep them cool and probably they will go into care because they are these are only juveniles and they all are also quite underweight," Ms Pope said.

However, she said bats can carry disease and should not be handled by the public.

She said any donations of fruit to the volunteer organisation would also be appreciated.

"There's not a lot of water out there," Ms Pope said.

"They do skim in the river, but some people have been pulling bats out of the river this morning — so it's just dreadful.

"It's going to get worse and worse, the stress is going to build up there are already some dead ones apparently hanging in trees and by the end of the week, there will be a lot of dead bats."

An eastern brown snake cools off at the beach in Port Noarlunga, in Adelaide's south. ( Supplied: Snake Catchers Adelaide )

Rangers spraying flying foxes in Melbourne

East of Melbourne's CBD, thousands of grey-headed flying foxes are also being sprayed by rangers and volunteers to keep them alive as the heat soars.

Parks Victoria rangers and volunteers are spending the day spraying the animals at Yarra Bend Park near Clifton Hill.

"We're particularly worried in December because of the number of young," Stephen Brend from Parks Victoria said.

"Peak birth month is October and we can see a number of mums carrying the baby, and if they [get] too hot today the mums could get distressed and die and the babies, they are just too vulnerable.

"They are absolutely crucial. They pollinate all the eucalyptus — all our commercial hardwood."

Parks Victoria ranger Stephen Brend sprays flying foxes with water at Clifton Hill. ( ABC News: Iskhandar Razak )

The grey-headed flying-fox — also known as the fruit bat — is vitally important for ecosystems because it helps pollinate forests.

Populations of the species have dropped by 95 per cent since European settlement and thousands of the species were killed this year during the NSW bushfires.