MELBOURNE, Aug 18 (Reuters) - Shares in BHP Group Ltd. (BHP.AX) and Woodside Petroleum fell on Wednesday as investors digested details of the Perth-based oil and gas group's $40 billion merger with BHP's petroleum arm, with some questioning the value of the deal for Woodside.

While a 6% fall in BHP's share price was linked to a decision to end its UK dual listing, where its shares have traditionally traded at a large discount, a fall of up to 4% in Woodside reflected concerns about the expansion, they said.

"It may be difficult to get a vote across the line, with Woodside shareholders likely to question the value of the merger," said Jamie Hannah, deputy head of investments at Van Eck Australia, a shareholder in both BHP and Woodside.

"Woodside is one of the worst-performing companies within the energy sector globally post-COVID; the company doesn't yet have a strong mandate to enter a deal of such questionable value and this could further drag on Woodside's shares," he said.

BHP agreed to hive off its petroleum business to Woodside in a nil-premium merger, in return for new Woodside shares which will go to BHP shareholders, who will own 48% of the enlarged group.