I am going to be setting up a CCTV network and don't want to be using the AV cables. There is already CAT5E cable already laid and I was wondering whether I could send the camera video and audio outputs through the CAT5E cable
Only if the camera system is designed to run that way many older ones are still analog. There are adapters that will convert but then it likely not going to be compatible with whatever control unit you are using.