PTT told the Thai exchange the Federal Court of Australia had approved the takeover, under which PTT Mining Ltd (PTTML) would acquire all of the shares in Straits Resources, renamed International Coal Holdings Ltd (ICL), for A$544.1 million ($546 million).

The company said all the shares in ICL had now been registered in the name of PTTML.

PTT said in November last year it was buying 100 percent of the Australian mining firm as part of its long-term strategy to focus on high-quality coal assets with good growth prospects.

PTT Mining is a wholly owned subsidiary of PTT International Co Ltd, a holding company set up to invest in overseas businesses for the Thai energy giant.

State-owned PTT, Asia-Pacific's third-largest listed oil and gas firm by market value, has been looking to snap up foreign assets in a bid to diversify risk and bolster its presence in the international market.

PTT spent $335 million in April 2009 to buy part of Straits Resources' coal and salt assets, its first foray into the coal business and its second investment in Australia.

The company had said it expected to boost its coal output after the Straits Resources and other acquisitions to 30 million tonnes in the next five years.

At 0334 GMT, PTT shares were up 1.5 percent at 342 baht while the broader market was 1 percent higher.