I can think of a couple of reasons, anď I'm just playing devil's...

I can think of a couple of reasons, anď I'm just playing devil's advocate here:
1. The gathering lines lead to a processing area for removal of minor impurities before being temporarily stored in compression tanks. These tanks have their own pipes which lead to Kincora, the main processing plant, and/or the Newstead Storage Facility. This then goes through PPL 3 to Wallumbilla. The gas currently being recorded at well head, some 200-400mscf/d (or around 0.3tj/d) is possibly in these holding tanks, therefore not recorded at the Wallumbilla connection.
2. More unlikely, the gas is being semi processed and stored in the Newstead Facility, but I don't know why this would be. I know they are trying to partner/sell/lease Newstead so possibly running gas into it would check it's pipelines, connections and regulators.
In my opinion the first option is most plausible in which case the numbers will trend up on the AEMO GBB.
Current flows through Wallumbilla are 5tj/d.