It's got nothing to do as to whether the aircraft or crew are low vis capable (99.9% of the time they are BTW)
It's got everything to do with legally being able to continue past the latest point on your flight plan (we call it DPA) with sufficient fuel to carry Melbourne (above alternate minima)
Everyone knows you can get in on a 0/0 Cat III approach (with no Decision Height seeing nothing at all), but you must be legal fuel wise from DPA to actually go to Melbourne commence the approach. They weren't, they went to Adelaide (gas up, carry an alternate for Melbourne, fly to Melbourne, carry out a low vis approach in Melbourne if still poor weather, land, buy duty free, go to hotel!)