Nauru, which is home to an Australian immigration detention centre, is hosting this year's leaders' summit, which will get under way on September 1.
Amnesty International has co-signed a joint open letter representing a coalition of 84 NGOs, which addresses Forum leaders and observers about conditions for refugees on Nauru, and also PNG's Manus Island.
Amnesty's Pacific researcher, Roshika Deo, said the system is clearly broken if children on Nauru are considering self-harm.