* Origin slashes APLNG stake value by further A$815 mln
* Writes down Lattice Energy business by A$357 mln
* Expects to complete Lattice sale by end 2017 (Adds background, details on H1 writedowns on APLNG)
Aug 10 (Reuters) - Origin Energy expects to book a post-tax impairment charge of A$1.2 billion ($947 million) at its full year results, mostly on its Australia Pacific liquefied natural gas (LNG) project, adding to a hefty writedown it took in the first half.
APLNG, operated by ConocoPhillips, is one of three coal seam gas-to-LNG plants which opened over the past two years in the northeastern state of Queensland amid a sharp slump in global oil and gas prices.
Australia's top power and gas retailer said on Thursday the charge on its 37.5 percent stake in the A$24.7 billion APLNG project is likely to be A$815 million, reflecting weaker oil prices, exchange rates and costs.
Origin booked a A$1 billion impairment charge on APLNG in the first half of the financial year, which at the time came as no surprise as rival projects had done the same.
Origin also expects to book a A$357 million writedown on the value of its Lattice Energy gas exploration and production arm amid a sale process, which the company said it expects to complete by the end of 2017.
The first and second half writedowns are expected to result in a total charge of A$3.1 billion for the year, the company said, adding the charge would have no impact on cash flow and underlying core earnings.
Origin Energy's full year financial results are due on August 16. ($1 = 1.2671 Australian dollars) (Reporting By Rushil Dutta in Benagluru; Editing by Sonali Paul and Richard Pullin)