We had a big spike in our electricity bill as well. Whilst it wasn't as high as yours, ours is usually around 300 in the middle of winter. Our recent one was almost $600! Almost doubled. I couldn't work out why because I had not used the dryer or the aircon/heating over the past 2 months whereas the two months prior to that in the middle of winter when I was using those things liberally, our bill was only $330. I rang Synergy and all she said was that my meter could be running fast and that it would cost me $150 for them to come out and test it! Um no thanks. We've been monitoring our meter and it's definitely not running fast. We can't work it out. Must be some conspiracy happening!