Foundation for National Parks and Wildlife spokeswoman Leonie Gale said microbats needed to be moved from inside houses to alternative roost sites outside, where they could continue to feed on insects.

``Bats are eating as much as 40per cent of their own body weight in a single night or several hundred insects per hour,'' she said.

``Many of our microbat species are hollow-dependent which means they live during the daylight hours inside the hollows of trees or branches.

``Competition from birds, possums and gliders, along with the clearing of many old trees, means microbats may find the roof or walls of your home the perfect roosting place.''

She said the smallest microbat weighed three grams and could slip into gaps as small as five millimetres wide.

``Castle Hill microbats are fully protected, which might raise the issue of offences and penalties if any are harmed.''

If you find a microbat that may need help, call a wildlife rescue service for advice.

Details: batrescue.org.au or backyardbuddies.net.au.