Hi Sam1990!
Welcome to AGL Community!
High gas bills can be very stressful, good news is there are a few things you can check yourself to see what is happening.
First thing we recommend is checking the meter reading on your meter and confirming it is in line with the "End Read" on your invoice, if that is not the case, contact us and we can review the bill.
Monthly billing can cause a discrepancy if the last bill was an estimate as this may be a catch up bill for the gas that had not been settled, to avoid this you can provide meter reading through your app or online account every month and we will use those to bill you.
Thank you
Ellen