MELBOURNE (REUTERS) - Mining giant BHP Billiton reported a record US$6.4 billion (S$8.58 billion) annual loss on Tuesday (Aug 16), hammered by a bad bet on shale, a dam disaster in Brazil and a commodities slump, but said it expects its free cash flow to more than double this year.

"While commodity prices are expected to remain low and volatile in the short to medium term, we are confident in the long-term outlook for our commodities, particularly oil and copper," chief executive Andrew Mackenzie said in a statement.

Even excluding US$7.7 billion in writedowns and charges, underlying profit slumped 81 per cent to US$1.2 billion for the year to June 2016 from US$6.4 billion a year ago, hit by weak iron ore, copper, coal, oil and gas prices.

The underlying profit was the weakest since the merger of BHP and Billiton in 2001, but better than analysts' expectations of around US$1.1 billion.