Hundreds of dead bats were left scattered on the ground and dangling from trees after extreme heat decimated a Sydney colony on Sunday.

While most Campbelltown residents did all they could to escape the heat, several WIRES volunteers descended on the colony near Campbelltown train station as part of a mass rescue and body collection mission.

Bats from the Campbelltown colony died in Sunday's extreme heat. Credit:Help Save the Wildlife and Bushlands in Campbelltown

At last count, 204 dead bats – mostly babies – had been collected, Campbelltown colony manager Kate Ryan said.

"I don't know how many times I bent down and got on my knees to pick up a dead baby," Mrs Ryan said.