It is nocturnal and has a distinctive "oom oom oom" call. It sounds a bit like a motor that won't start.
The Birds in Backyards website has some great details on the Tawny Frogmouth including its call.
It is a common misconception (which I for one had) that a Tawny Frogmouth is also called a Mopoke and makes the plaintive "mopoke" call at night.
In fact a Mopoke is a Southern Boobook Owl and it makes the call.
The brilliant Owl Pages web-site explains this.
The Tawny Frogmouth is not an owl - being more closely related to a Kookaburra.
It is about 50cm from head to tail.
The male (as shown here) has greyer plumage; the female's is browner.
These photos were taken in Cheltenham in Sydney.