Operator Origin (37.5%) is joint venture partner in APLNG along with ConocoPhillips (37.5%) and Sinopec (25%), with the export concern also supplying 30% of the east coast domestic market last year.
The ACCC said it had "considered the effect of the acquisition on domestic gas supply and the level of competition between suppliers of domestic gas" and found in favour of the transaction.
Ironbark holds some 0.34% of total east coast gas reserves at some 134 petajoules of 2P reserves in the Surat Basin, according to the ACCC, which is higher than Origin's recent estimate of 129PJ but far below an earlier estimate of 249PJ made when Origin purchased the field a decade ago.
"Origin will derive value from the development of Ironbark through its investment in APLNG," CEO Frank Calabria said in February when announcing the transaction.
"We had regard to the relatively small size of the Ironbark project. We also considered the alternatives available to Origin to either sell Ironbark to someone else or develop the project itself," ACCC commissioner Roger Featherston said today.
"In our view, neither of these alternatives would lead to a significantly different outcome for domestic gas users from that of the sale of Ironbark to APLNG."
Featherstone noted the watchdog had "long voiced concerns about the challenges facing the east coast market".
The ACCC's chair Rod Sims said in March at a Sydney gas conference he had "no sympathy" for east coast LNG exporters who were feeling the tightening of the noose as they balanced domestic demand with export contract obligations.
"They brought this on themselves. I have no sympathy," he told the room.
Sims said then that years ago he had been sceptical of the three separate Queensland LNG projects' abilities to meet export commitments and keep the market supplied at a reasonable price.
"There is almost contempt for the domestic market."
The ACCC has also blamed Victoria and New South Wales' refusal to commercial onshore gas reserves or allow drilling for possible future gas shortages and higher prices.
Featherstone said today it will "continue to closely examine the acquisition of further gas reserves by major LNG producers and the likely impact on competition".
APLNG's exports was one of Origin's largest revenue drivers last quarter thanks to a higher oil price despite cargoes remaining level over the previous quarter.
Ironbark, not to be confused with a large offshore Western Australia project BP, Beach Energy, New Zealand Oil & Gas and Cue Energy plan to drill next year, has lain undeveloped for some time with Origin previously suggesting it may sell or farmdown.
With Origin still in the driver's seat, but the consortium backing the project, it can process the gas straight through plant, rather than negotiating tolling with its partners.
Origin acquired 100% of ATP 788P for $660 million from the Pangaea Group of Companies in August 2009 when the Queensland CSG boom was running at full throttle after its Australia Pacific LNG joint venture declined to exercise its options to acquire the interest.
Last year it cut $500 million in value from Ironbark, lowering expectations on the once-prospective project and booking a $355 million after-tax impairment.