Justinmetals is right - it's not about whether the molecules actually go past Mt Isa or not. Once the molecules are in the pipeline network they could end up anywhere. CGP is currently flowing north but could easily swap around temporarily (for example if one of the big users in Mt Isa shuts down for maintenance), in which case CTP's molecules could well end up in Sydney despite being sold to a buyer in Brisbane.
It's about where CTP has contracted with the buyer and the pipeline operator to deliver gas to. If it's Wallumbilla, then it doesn't matter if the molecules don't go past Mt Isa, CTP pays pipeline tolls to Wallumbilla. That's why backhaul arrangements are so critical, so that CTP can sell gas to a buyer in Brisbane but only pay to haul gas to Mt Isa. Both CTP and IPL benefit - without backhaul, the only beneficiary is the pipeline operator.
