Outgoing BHP Billiton chairman Jac Nasser has conceded the global miner's $US30 billion investment in US shale oil and gas assets was a mistake, saying the timing was "way off" but defended the move as the right decision at the time.

Mr Nasser, who will be replaced as chairman by former Amcor chief Ken MacKenzie in September, also said ideas proposed by activist investor Elliott Management were unsustainable and the company had a responsibility to its shareholders to say "thanks but no thanks".