What bothers me is the possibility of someone joing a flight at a regional non sterile airport and bypassing security at Melbourne or Sydney.
My other concern is the lax way aircrew appear to be treated by securtiy and immigration. I believe there may be a signifigant risk of impersonation. I don't think anyone could actually get on to an aircraft that way, but they could bring stuff into the sterile area and hand it to someone else.