Woodside Petroleum's first half profit has fallen slightly because of the costs of starting up its massive Pluto liquefied natural gas project.

Woodside's net profit of $US812 million ($A777.67 million) in the six months to June 30 was down from $US828 million in the previous corresponding period.

Underlying net profit, which excludes one-off costs associated with Pluto, was $US865 million ($A828.43 million) in the six months to June, up from $US828 million in the previous corresponding period.

Woodside owns a 90 per cent stake in Pluto, which was commissioned in March this year.

The company today said exploration drilling at Pluto had failed to uncover enough commercial gas to bring on a financial investment decision for its expansion.

"Nonetheless, discussions with other resource owners regarding potential Pluto expansion are active and are expected to continue through 2013," Woodside chief executive Peter Coleman said in a statement.