Under the deal, APLNG, which is held equally by Origin Energy and ConocoPhillips, has agreed to sell about 190 petajoules of gas from ATP 648P and ATP 620P during an initial ramp-up period of about two years to BG.
This will be reduced to about 25PJ per annum from ATP 648P over the balance of the initial 20-year contract.
BG also holds two options to extend the contract by five years each.
"The transaction evidences the importance of project cooperation. It supports the development of both Australia Pacific LNG and QGC's LNG projects by assisting in the efficient development of reserves and management of gas production," Origin Energy managing director Grant King said.
Origin added the start of gas sales would coincide with the start of commercial production at QCLNG in 2014.
King noted the agreement opened an "export channel to market" for part of APLNG's gas resource and would bring forward its monetisation.
QCLNG will be an 8.5 million tonne per annum development while APLNG remains the largest planned CSG-LNG project with expected capacity of 14-16MMtpa.
The deal comes as Origin posted a 28% increase in underlying profit to $355 million for the half-year ended December 31, 2009, over the previous corresponding quarter.
This was due to reduced financing costs which in turn reflect lower net debt due to funds from the APLNG farm-out to ConocoPhillips as well as a $20 million increase in underlying EBITDA (earnings before interest, tax, depreciation and amortisation) to $706 million.
Revenue for the half-year was also up by $84 million to $4.3 billion while net profit after tax was $731 million compared with the $6.7 billion in the previous corresponding period, which had included a gain on the sale of APLNG interests.
"During the past six months we have invested significantly in the ongoing growth of our business with capital expenditure on growth and acquisitions of $1.5 billion," Origin chairman Kevin McCann said.
King also reiterated an earlier forecast that full-year profit for 2009-10 would be 15% higher than the previous year.
"Looking further ahead, Origin has a number of opportunities to continue to grow and develop its business," he added.
"Origin expects to increase its share of growing energy markets in Australia and New Zealand.
"Australia Pacific LNG will continue to develop its CSG-to-LNG project. We can see potential additional gas and oil discoveries from an expanded exploration program."