Hi @Gasbag , the "estimation" is made when the bill is issued, so you receive a notification via email of your bill, and you'll be able to see on the bill whether it is estimated or actual. You can then enter a self-service read if you want the bill to be amended. In any case, it will be automatically amended the next time your local distributor provides a read.
Alternately, through your My Account or the AGL app, you can see when the next bill will be issued, so you can provide a read shortly before that date to ensure that it is accurate to your actual usage.