I had the same issue 13 years ago...took them nearly a year to send the first bill then they expected it paid in full immediately. They kept saying it was an IT department issue which was a well known fault and that it would get sorted eventually I just had to wait, maybe for a few more months.
That they still don't seem to have fixed this problem is disgusting but AGL IT department are a law unto themselves. Overseas and virtually unaccountable even to very senior managers (believe me, over the years I have tried over various IT issues). If AGL could get their IT issues sorted once and for all by bringing the IT services onshore to Australia and get proper control over capable staff, it could leave the competition behind, but they just won't address this glaring, ridiculously longstanding and very central issue. Heads firmly in the stand. Customers need to start speaking up about this.