Check your recent bills to see if they have been 'actual' meter readings or 'estimated' meter readings. Your supplier is required by Aus law to state on the bill which it is. Possibly they have underestimated your last few readings and now have an actual reading - meaning you have to pay for the amount they underestimated. Does that make any sense??
We have just been through exactly the same thing with our gas bill. Went from about $300 to a bill of $1500!! Huh??? Turns out they had been estimating our bill for a whole year. If you are not satisfied with the response of your supplier when you query the bill, contact the energy ombudsman in your state. They will act as an intermediary between you and your supplier to investigate the bill on your behalf. We had the ombudsman investigate and (after a whole lot of confusion and argument!!) we did not owe the gas company the amount they had stated (they sent the $1500 bill based on their 'estimation' that they had 'underestimated' the original bills... they still hadn't taken an actual meter reading!).
Good luck, I hope you get the bill sorted and you don't owe them so much money!!