I had a search and confirmed gas has a supply charge, and unless the service is actually disconnected officially, you will still get a supply charge monthly.
See here https://www.agl.com.au/content/dam/digital/agl/documents/pdf/agl1059-bill-explainer_resi_gas_draft4....
Your bill should show you this, or if bill readings show that there has been usage, and your actual bill reading is changing upwards each bill, either the meter reader is reading the wrong meter for your property (a unit / flat / apartment ?), or they might be estimated readings.
Again, your bill should show this.
A call or chat to AGL would be quicker than here most likely, but usually someone form AGL comes along here and says they will message you for more details, and investigate for you.
Sometimes they actually do that.
Contact them for more speedy resolution . . .
https://www.agl.com.au/help-support/contact-us?cidi=A11062