SYDNEY - Communities around Rio Tinto's shuttered copper mine in Bougainville face a serious threat of flooding due to unstable mine infrastructure, an interim report commissioned by the mining company said on Monday.

Due to the urgent nature of the threat, a rapid risk assessment will begin in coming weeks by global environmental firm Tetra Tech Coffey, which prepared the report, and will include on-ground inspection to verify the report's findings.

Rio Tinto reached an agreement with the Bougainville community last year to fund an environmental and human rights impact assessment of the Panguna copper mine that it ran until 1989, following a complaint brought by community members who were represented by the Australian human rights group Human Rights Law Centre. 

Monday's interim report said a levee at the junction of the Jaba and Kawerong rivers, constructed at the time of the mine's operation, "is almost certain to collapse at some stage in the future".

"Structures and people that live on the floodplain downstream of the Jaba River would be directly impacted by flooding or landslide effect," it said.

It it was not possible to predict when the levee might fail or how severe the failure might be, it said, due to limitations of current information.

The report said the rapid risk assessment process would now be led by the local authorities in the Autonomous Bougainville Government, with the support of Rio Tinto and the Human Rights Law Centre.

Rio Tinto said in a statement to Reuters that community representatives in Bougainville had been advised of the interim findings and the work being undertaken to better understand the risks and mitigations.

A Rio Tinto spokesperson reiterated that a formal impact assessment would commence on the ground later this year, which "will provide all parties with a clearer understanding of the impacts, so that together we can consider the right way forward".