No Traffic - In Australia, that would be a waste of resources.
However ICAO requires that instrument approaches are contained within controlled airspace - another Australian non-compliance. Here we require that they be contained in the same category of airspace, but, CASA hands out exemptions all over the place, particularly for NDB approaches.
- A crazy example exists at Toowoomba where there is an RNaV approach that flies near Oakey. When the Army Tower is open it cannot be used!
In Europe an instrument approach can only be used when a tower is operating, that is the reason behind their push for remote Towers.
In the USA Class E airspace with a CTAF for VFR traffic is used. Whatever your views about Class E, ICAO considers it to be controlled airspace, so it meets the international criteria. The US system is far more applicable to Australia than the European.
So Class E, controlled from Melbourne Centre by en-route controllers, would have provided separation of the two IFR training aircraft. Instead the same Melbourne controllers with Class G have their work cut out, giving traffic information, listening to what the pilots do next, re-evaluating the traffic information and re-issuing it, and so on....
Believe me it is sometimes much easier to separate than provide ever changing traffic information. (PS. That is also why Class D Tower ATCs sometimes "over separate" VFR aircraft, it is quicker and easier than passing traffic information)
I hope that answers your question.