(Adds details) Australian oil producer Woodside Petroleum Ltd's...

(Adds details)

Australian oil producer Woodside Petroleum Ltd's (WPL) agreed merger with BHP Group's (BHP) petroleum arm is in the best interest of its shareholders, an independent expert said, valuing the combined group at around $40 billion.

Global miner BHP agreed to hive off its petroleum business to Woodside last year in a nil-premium deal that will give BHP shareholders a 48% stake in the combined group and turning Woodside into a top 10 global independent oil and gas producer.

Accounting firm KPMG said its assessment found the merged group would have an underlying value between A$26.25 and A$29.81 a share, which was equal to or more than its valuation of Woodside's current per share value.

"Based on these measures, the proposed transaction is, in our opinion, fair to Woodside Shareholders," KPMG said in a report released to Woodside shareholders on Friday ahead of a vote on the deal on May 19.

Woodside unanimously recommended its shareholders vote in favour of the merger.

"BHP Petroleum's asset base provides Woodside with immediate access to significant development and growth opportunities, within a time frame that is unlikely to otherwise have been available to Woodside as a standalone entity," KPMG said.

In a separate announcement, BHP said based on Woodside's share price of $25.55 on April 6, the implied value of BHP Petroleum is $23.4 billion.