The Australian domestic natural gas market isn't big enough to justify investment into the exploration and production for the local market only.
If we didn't allow export of the large proportion of our gas reserves, gas would either be really expensive here or non-existent since no-one would bother investing to produce.
Melbourne only gets gas because Bass Strait oil is nearby. If the Bass Strait oil production was any further from civilization, it'd just be flared off, such is the nuisance of it.
Ironic that a 10-buck gas program comes from the bloke who thinks this nation can't sustain a much larger population. Without our resource exports, we couldn't sustain the micro population we have!