Lawyers in Solomon Islands are launching the first court challenge to the immunity of Regional Assistance Mission police.
John Kwakwala Makasi alleges police forced him to urinate in front of them during an interrogation.
Police held Mr Makasi at the RAMSI police base in Honiara as part of their operation to catch the killer of a colleague, Adam Dunning, last year.
Mr Makasi's lawyer, Charles Ashley, says Australian Federal Police sent to investigate the claim haven't admitted liability, and he will file against RAMSI on Thursday.
"It is a civil claim brought under the constitution of Solomon Islands. They are immuned, from the intervention legislation which they have come under, they are immune from a lot of things, but not the constitution."
Mr Ashley says he understands two Australian police officers to be the focus of the internal investigation.