Peter Rodd, president of Port Stephens WINC (Wildlife In Need of Care), said the mass eradication was first reported on Tuesday by a Raymond Terrace resident walking through the park.

"What we discovered when one of our carers arrived later that afternoon was an absolute disaster ... it was a horrific scene of hundreds of dead baby bats and it was very distressing for many of our members."

Mr Rodd said by Wednesday the death toll had reached well in advance of 500.

"But we have managed to save many hundreds as well," he said. "Our problem now is to find more volunteer carers. It has reached crisis point."

Mr Rodd said at first it was believed the flying foxes had died of heat stress "but after realising the vast majority were babies it is now believed that the mothers have abandoned their young due to a lack of water and food".

"Partly due to the heat and smoke from nearby bushfires, the new mothers are struggling to keep hydrated and cannot produce the milk to feed their young, so they abandon them," he said.

Mr Rodd said that carers had worked tirelessly overnight Tuesday feeding and sustaining more than 100 baby bats they had managed to rescue from almost certain death.

He said that the rescue mission had been a collaborative effort and that WINC had received generous support from a number of organisations including Port Stephens Council, NSW Health Department, National Parks & Wildlife, FAUNA (Fostercare of Australia's Unique Native Animals), Bunnings Heatherbrae, McDonald's Raymond Terrace and members of the public.

He said that the council had closed off sections of the park to the public.

"There is a health risk for anyone handling the animals, so we ask anyone wanting to help to contact us first at wincenquiries@gmail.com."

The WINC hotline number for anyone finding any animal in distress, ill or injured is 1300 946 295.