Hey SeamFiend,
Where did you get the $2-$3GJ gas margin from?
Here's some interesting info on Aus pipelines, transmission grid and tariffs.
http://www.accc.gov.au/content/item.phtml?itemId=904614&nodeId=67a053a813b3db39f76965af4451dfea&fn=Chapter%209%20%20Gas%20transmission.pdf
I'm assuming the scenario you're referring to below is for sending gas North from ESG's fields?
JT