I was told that the reason only registered or certified airfields can have instrument approaches is because of NOTAMS. ALA's like Goolwa cannot issue NOTAMS unless we get CASA approval each and every time, hence why most, if not all, ALA's are PPR (Prior Permission Required). So the reasoning by CASA is that because an ALA cannot issue a NOTAM then a pilot under IFR will not know if there is an issue with the runway etc. and therefore is unsafe*. CASA believe it is safer* for pilots to scud-run into an ALA.
* Safer in this context is synonymous with liability, i.e. If CASA allows approaches into non-registered/non-certified airports then CASA is liable, If a pilot crashes while scud-running its not CASAs fault.