Mr Truss said it was also of concern that some pilots had not submitted applications at all
I don't know why he's surprised at that - from the CASA website:
You need a valid ASIC if you need frequent access to a secure area of a security controlled airport that has Regular Public Transport (RPT) operations.
If you don't need frequent access then, from the DOTAR's website,
http://www.dotars.gov.au/transsec/aviation/fact13.aspx, you can get a VIC - visitor identification card, supervised access.
Finally,
Holding an ASIC card does not give you a general right of entry into the secure areas of airports. You can only be in a secure area if you have a work-related reason to be there.
Very few private pilots would have a work-related reason so what's the use in getting an ASIC?