Considering most Oz airports are not security controled airports - what is it that ASICs are supposed to acheive.
I think you will find most airports at major centres are in fact security controlled. Sure there are the remote dirt or sealed strips around the place but also without fuel or any other infrastrucucture there. If you think you can survive without an ASIC and some have, fine don't get one. But don't complain when you get rumbled by the local airport Nazi when you drop in to get some AVGAS.
Passengers boarding either comercial or private flights do not need ASICs.
Correct, but they must be escorted AT ALL TIMES whilst airside by an ASIC holder.
As has been pointed out, the people tasked to do the checking of ASICs at many airports are well known. Not hard for osama the bad guy to work out when best to enter the airport.
As long as said bad guy is on the airport during a non operational period, and cannot plant something that can be loaded onto or damage an aircraft... then so what? Procedures, which I will not discuss, for the protection of aircraft and pax during operational periods are a different matter.
The costs of having a representative of the airport do the card checking is added to the airport running costs - higher landing fees and ground rental costs.
I would suggest the cost of all the other security compliance measures that have been supported on other threads such as cameras, fences, screening eqpt etc will have a far greater impact on an airports operating cost.