The absolute reality of the situation is that any major retail fuel supplier will dick around with the percentage of ethanol in the mix. I don't even think it'll be called E85 at the pump. Probably 'high ethanol unleaded' or similar.
Whether an argument for a seasonal mix exists in Australia (I don't believe it does), the fact of the matter is that fuel companies will use the cost of ethanol at any given time and their ability to vary the ethanol content to leverage the wholesale cost of unleaded petrol.
In short, the mix will almost certainly change between 70% to 85% without warning and you won't know when or where.