More than 700 flying foxes have died during a heatwave in the New South Wales Hunter region town of Singleton, with many of their corpses still hanging from trees.

The mass death at the Burdekin Park colony began Friday and peaked as temperatures hit 47C over the weekend, Wildlife Aid Inc bat coordinator Jaala Presland told 9news.com.au.

Video shows the native animals' lifeless bodies hanging upside down from trees and littering the ground of the town's central park.

"We had half a dozen [live bats brought in] on Friday evening. Saturday we knew the temperatures were looking high again and we had 80 come in, and the death was sort of starting to tally," Ms Presland said.

Some of the bats were found lifeless hanging from the trees, while others littered the grounds of the town's central park. (Supplied)

"The death toll is currently sitting around 700 that's just from the park currently and very close surrounding areas."

Wildlife Aid Inc had put a team on alert in response to the extreme heat forecast, and other care groups have since travelled to Singleton to help out.

The carers must now treat dozens of affected bats, and collect and dispose of hundreds of bodies over the next couple of weeks.

"They come in and we need to rehydrate as quickly as possible, their bodies need to be cooled down relatively slowly so they don't go into shock and then we transfer them out to different care groups," Ms Presland said.

She said there was generally a "small heat stress drop" most summers of about 50 animals, and some deaths during winter and hail storms.

More than 700 animals were found dead. (Supplied)

The weekend's toll was the highest since 2500 flying foxes died at the colony in 2004, Ms Presland said. About 2000 bats were counted at the colony before the weekend, and the population now stands at 600.

Most of the dead were grey-headed flying foxes, with some black and little red flying foxes also succumbing to the heat.

The deaths of little reds was unusual because "generally they cope with higher temperatures", Ms Presland said.

"To a certain degree it is a natural event, however they're not in a natural environment due to human disturbance," she said.

"In a normal camp you'd have canopies and they'd be able to get down low."

During heatwaves, the flying foxes are faced with staying in the colony and effectively cooking in their roosts or contending with predators if they try to fly away, she said.

Ms Presland said the weekend temperatures were some of Singleton's hottest on record.

The Bureau of Meteorology's nearby Cessnock Airport weather station recorded 46.2C Saturday afternoon, and 45.1C yesterday afternoon.

The flying fox colony was established in the town's central park in 2000, and some trees have been removed since that time. Its population varies greatly as bats frequently come and go.

Grey-headed flying foxes conservation status is listed as vulnerable in New South Wales and across the country. They are the largest Australian bats and are found from Queensland to South Australia.