Mystery fees are never a pleasant surprise! This is likely a pass-through fee charged by the local distributor in your area - the company that maintains the pipes, wires and meters.
To check out the reason for this fee you can use our fees and charges tool: https://refer.agl.com.au/fees/?cidi=A10284
If you require more information, log in to your account and click "Message Us". An agent should be able to provide more information about the specific fee.