dvahillandpilot, I know this might sound a bit like a Tin Foil Hat Wearing Conspiracy, but what would stop you from flying into a Regional Airport like Dubbo with a bomb in your bag or on your person, walking into the sterile area and handing it off to someone else who then jumps on the flight to say Sydney before jumping onto something much larger within Sydney Airport as they are transferring Domestic?
It sounds a bit loopy but if you don't keep one Sterile Area "Sterile" then you've effectively made any connected Sterile Areas also not "Sterile".