why do we need mufflers? Its all about noise restriction. Don't have it off the top of my head, but there will be something like a 90dBA noise level for cars at the exhaust that your vehilce should be meeting. Whether your vehicle can meet the noise level requirements listed in your State with or without a muffler is dependant on what mods you've done to your vehilce, how well your engine runs. Tehre are Australian standards on the correct procedure on how to test for engine noise (e.g. set distance away from the engine or exhaust, what speed to run at and what rev range). In short no quick answer.
Cats are required in all States and Teritories.