Late on Tuesday Banpu said its wholly owned Banpu Minerals (Singapore) Pte Ltd would spend A$45 million ($45.4 million) to buy the stake, which will give Banpu access to coal resources in Mongolia and enhance its long-term outlook.

At 0336 GMT, Banpu shares were up 1 percent at 744 baht after hitting their highest since Feb. 24 at 748 baht. The broad market was 0.5 percent higher.

This is Banpu's second acquisition in Australia after it bought Centennial Coal Co Ltd for $1.9 billion last year as part of a regional expansion drive.

"We believe this is a good move for Banpu, with the coal resources at Hunnu's two mines equivalent to 1.5 percent of Banpu's current resources," Kim Eng Securities said in a note.

Kim Eng said it calculated the acquisition price per tonne at US$3.7, which was less than the US$5.7 per tonne paid for Centennial Coal because Hunnu was still developing projects and had not launched operations.

The top broker rated Banpu a "buy" because it expected strong earnings growth this year.

Banpu, valued at $6.6 billion on the Thai bourse, is looking overseas because its local coal reserves are depleted.

It is already Indonesia's fourth-largest coal miner with five mines there.

Along with Australia and Indonesia, it is also targetting South Africa.