In IMC it would be extremely difficult - separation becomes more luck than good management.
Considering that that is how uncontrolled airspace has worked in Australia since day one, I would say that it is more by good management than luck - good management by the pilots concerned. Almost all mid airs have occurred in VMC between VFR's
Without Class E Australia is basically relying on low traffic densities to ensure that you don't get multiple IFR aircraft trying to do the same thing at the same time. Every so often, that doesn't work out. According to the report CASA calculated the collision risk as 1 in 7200 years - I would be interested to see the assumptions behind that.
We don't have Class E,
because we have low traffic densities. Realistically, how often do 5 IFR aircraft in IMC actually try to arrive at the same place at the same time in Australia? OWN was infinitely more likely to fly into a hill than into another aircraft, based on this country's accident history.