You can give them a call to find out.
If you are lucky, you might not need to pay for it at all, since after a certain period (not sure how long), you won't be liable for the old bills if it's never billed to you correct me if I am wrong.
Hi BZaurora and aurorafrog,
See my post here. http://forum.homeone.com.au/viewtopic.php?p=789539#p789539
Hi ,
I had this happen to me in the last place I was in, they had taken my meter details incorrectly and were billing the neighbors. By the time I contacted them I didn't have to pay as they had written off the last 9 months so I only paid a fraction of what I owed.
The limit for backpayment is 12 months or 9 months depending on the situation and you can demand a payment plan when they finally come up with a bill.