Well, if it's opinions you're after ...
US jungle greens were sometimes worn on an individual basis by Australian Forces in Vietnam. This might have come from then. The camo job may be as a result of required specialist role i.e. recce (we're talking battalion recce assets here, not SASR - they had ample access to all sorts of camo).
The other possibility is that it was used after Vietnam. As a whole, the Australian Army did not introduce camouflage until the mid-1980s. Before that only 'Special Action Forces' i.e. SASR and 1 CDO Regt wore cams (from around the free world) on a regular basis. However, once again, amongst the regular forces, individuality could assert itself depending on the tactical situation and this might have been used by any infantryman, perhaps on recce task, or on OpFor.
As it stands it is certainly an interesting piece and it would be worth chasing up more details. Did it come from Western Australia by any chance?