You're right, police do not check passports at the airport building, they're immigration officers - often they do have members who are trained police officers,

You're right, police do not check passports at the airport building, they're immigration officers - often they do have members who are trained police officers, but they're very rarely the people at the front dealing with the 300 people getting off a plane; the real officers are in the back who deal with the few hand picked out. This holds true for every country I've been in, and I know for sure actual police officers are only in the back in Canada and the UK (I'd safely assume Australia uses the same commonwealth practices though) as I've been directed back there a couple of times (entering Canada) and my wife was directed into the back (entering the UK).