Seems this is a common issue with gas.
This thread linked below is from 2023, and Olgi's answer on page one suggests dual gas metering in "buildings where Bulk Hot Water is used, meaning that one of the meters is used to measure your General Consumption and the other meter measures how many litres of hot water you're using at your apartment."
https://neighbourhood.agl.com.au/t5/Meters-Connections/Gas-bill-has-two-meters/td-p/28652
A newer post from last week still shows people have this as a general issue . . .
https://neighbourhood.agl.com.au/t5/Meters-Connections/Gas-bill-has-one-unknown-meter/td-p/36658
No answer given for that one apart from the usual "send a private message" to address the inquiry.
It'd be good to know outcomes and reasons for such things, so others can more easily find possible causes.