AC,
I seem to recall someone posting some stuff about this back when the NAS [wash my mouth out with soap] threads were running.
I think it was something to do with the fact that IFR aircraft dropping through the bottom of cloud in 'controlled' airspace - particulary Class C and D - needed to have a little time to orientate themselves and start searching for conflicting VFR traffic. This was also why the speed restriction of 250kts below 10000 feet was introduced. It's a bit rough on the poor old IFR drivers if they drop out of cloud straight onto a Cessna 152 with it's tail sticking into the cloud!
You'd need to look back through the threads - but I think this is the reason. Also, I think it came from ICAO - not an Australian idea.