Thousands of bats have dropped dead in the heat, falling from the trees as scorching summer conditions blasted south-eastern Australia over the weekend.

Among the worst affected areas are around Casino in northern NSW and Singleton in the upper Hunter, where residents and council workers say thousands of bat carcases have been discovered scattered on the ground in parks and along riverbanks.

Yet more dead bats remain clinging in tree canopies, and are expected to fall to the ground as they start to decompose over the coming week, Richmond Valley Council general manager Vaughan Macdonald said.

Mr Macdonald said it became clear about 3pm on Sunday that the bat colonies were in distress, as the mercury hovered around 46 degrees in the region.