The Refugee Action Coalition says there has been an outbreak of dengue fever in the Australian-run asylum seeker detention centre on Nauru.
The coalition's spokesperson, Ian Rintoul, says there are three confirmed cases among staff and detainees at the centre, and there is a serious risk of it spreading because of the number of people in a confined space.
Mr Rintoul says medical facilities are inadequate and some detainees who have presented symptoms of fever have just been given paracetomol.
"I think it has to be said that the medical facilities in the detention centre itself are completely inadequate to deal with dengue fever. I mean, even the hospital arrangements on Nauru itself are stretched to deal with those cases."
Australia's Immigration department is yet to respond to requests for information.