BHP Billiton shares fell on Friday as Brazilian police accused the mining giant's Samarco joint venture of prioritising production over safety.

Samarco, which is co-owned by BHP and Vale, is among three firms and eight employees accused of crimes after the conclusion of police investigations into the collapse of a Brazilian mine that polluted a major river and killed at least 17 people.

Samarco is accused of wilful misconduct, with police saying the company had ignored clear signs the dam was at risk of collapsing.

The company had skimmed on safety spending, focusing instead on increasing production despite obvious indications, such as cracks, that the dam was in danger of a breach, police said.

As well as Samarco, police accused Vale because it deposited its own mining waste in the dam, and VogBR, the service company that checked the safety of the dam.

Eight executives were also accused, although their names were not disclosed by the police.

The case now passes to prosecutors, who will choose whether or not to press charges.

Samarco has rejected speculation it was aware of an imminent risk of collapse at the dam, which held waste known as tailings from its iron ore mine.

"The dam was always declared stable," the company said, adding that increases to the dam's size were made in accordance with the project's design.

BHP Billiton said it, Samarco and Vale had commissioned an external study, led by global experts, into the technical causes of the damage failure.

"The panel's finding on these issues will be made public once they are complete," BHP said.

BHP shares dropped 81 cents, or 4.1 per cent, to $18.80, while Rio Tinto fell 3 per cent in a day of negative trading for the resources sector.

Brazil's federal court recently ratified a settlement reached by Samarco and Brazilian authorities over the deadly dam spill, which will see the company pay up to $US2.3 billion ($3.08 billion) over six years.

Samarco will also set up a fund for restoration work and to provide compensation for damages caused by the tailings dam spill.

Reuters/AAP