The bridge is in desperate need of replacement and the bats were found while Great Lakes Council was carrying out environmental studies as part of its timber bridge replacement program.

The bats, called Southern Myotis or Fishing Bat, are a rare species that roost in small groups in caves, mines, tunnels, culverts and other enclosed structures, usually near water. They eat flying insects and even rake the water's surface to catch small fish and bugs.

Cavities in timber bridges provide ideal homes for this species.

As a result, the council's ecological staff targeted the bridge with detectors that record the sounds of flying bats otherwise undetectable to the human ear. A colony of about 20 bats was identified.

The council's ecologist, Ryan Sims, who discovered the bats, said it was an exciting discovery.

"Fishing Bats are rare and fascinating creatures. They perform valuable ecological functions and, being listed as threatened, council has a legal responsibility to protect and manage them."

The Battles Bridge replacement is part of the council's Rural Bridge Program and the broader program to reduce the infrastructure backlog.

The council's project engineer, Scott Nicholson, said he had been guided by advice from the NSW Roads and Maritime Services, who have successfully relocated Southern Myotis bats from under timber bridges into purpose-built bat boxes under new concrete bridges in northern NSW.

"We will complete our environmental study and receive project approval. Then we provide an alternate roost in the form of a bat box nearby and close the roost cavity after the bats have left for the night's foraging."

The council will then provide permanent roosting boxes for the bats under the new bridge.