I thought they could only estimate one bill per year?
Nope, they can estimate as many bills as they please. Most providers will do you the 'courtesy' of trying to contact you after 12 continuous months of estimated readings to arrange an actual meter reading. The provider *should* attempt to gain at least one 'actual' reading per year. The onus is on the consumer to check whether the bill is 'actual' or 'estimated' and to contact the supplier should you believe your estimated reading is inaccurate. If you request a 'special' meter reading (ie. not the scheduled reading) in order to clarify whether the estimate is accurate or not, the supplier is entitled to charge you a fee (approx $60 to $100!).

What's more, if your supplier does underestimate your bill, there is no statute of limitation on how long they can chase you for payment. So if you live in a house for 6 years and don't realise you bills are being underestimated for the entire time you live there... the supplier can rightfully bill you for 6 years of their underestimation!! Apparently, it can and does happen.

I am in Vic and this information all comes directly from the Energy Ombudsman in my state - I assume other states are the same. As a consumer, it would seem to me that the rules and regulations are heavily stacked in favour of the supplier!! Don't we pay them to provide a service???