The further deterioration of mental health among refugees on Manus Island and the lack of psychiatric support "will lead to serious, adverse outcomes", the UN refugee agency says.
The agency, the UNHCR, made the finding in mission to the Papua New Guinea island last month.
It said its medical experts, a psychiatrist and general practitioner, observed "high levels of anxiety and depression" among about 580 men exiled to the island by Australia since 2013.
"A deep seated fear of being abandoned in PNG by Australia without adequate support has contributed to an acute sense of insecurity and helplessness," UNHCR said.
While refugees continued to be relocated to the United States, planning was critical for those who may be left behind, it said.
"The Government of Australia should immediately identify and secure alternate durable solutions outside of the bilateral arrangement between Australia and the United States of America, including acceptance of the continuing New Zealand offer.
"Clear information on all appropriate available options outside of Papua New Guinea should also be communicated to refugees."
In the meantime it could be "anticipated that an ongoing lack of support for vulnerable individuals will lead to serious, adverse outcomes... These negative consequences are clearly foreseeable and preventable," UNHCR said.
The suicide in May of Rohingya refugee Salim Kyawning underscored "the criticality" of mental health care needs being unmet," it said.
"Various reliable sources consistently advised UNHCR staff that coordination and planning for critical incident management and response had not been put in place with relevant local authorities, including the hospital and police forces.
"An intensive outreach effort to comprehensively identify vulnerable individuals and assess their level of current risk should be undertaken in a coordinated manner between service providers."
But confusion reigned among service providers, who "work in silos without clear information as to the role of others" despite being centrally contracted by the government of Australia, UNHCR said.
"Differing views persist between stakeholders regarding who has operational authority and responsibility for certain services... The Government of Australia has no continuous or regular on the ground presence to coordinate and supervise the fulfilment of contractual obligations by those it has engaged."
Of the company that recently took over running the refugee's small health clinic, Pacific International Hospital, the UNHCR found it was providing a service that continued to not meet the needs of refugees.
"Outside of the Pacific International Hospital clinic's hours of operation, primary care, emergency service provision and surge service provision in the event of any major health crisis are to be provided by the Lorengau General Hospital, with already very limited capacity to even provide care for Manusians. UNHCR staff and medical experts confirmed there is no attending anaesthetist, surgeon or psychiatrist available."
As well as calling for clear procedures to be established for the medical transfer of refugees "outside of both Manus Island and Papua New Guinea", UNHCR said outreach medical assistance was critically needed for those with mental health concerns, "who may be unable or unwilling to engage proactively with service providers.
"Medical care for those who do not speak English is seriously compromised by the complete lack of on-site interpreters... Trauma and torture counselling services, along with necessary interpretation services, should be immediately reinstated on-site at all three accommodation facilities."
The UN Mission found 287 refugees at the East Lorengau Refugee Transit Centre, 175 refugees at West Lorengau Haus and 130 asylum-seekers in Hillside Haus.
While enough toilets had now been installed in the facilities, UNHCR noted an absence of smoke detectors and fire alarms at both Hillside and West Lorengau Hauses.
"Following the fire at Hillside Haus on 20 May... various sources advised UNHCR that the refugees and asylum-seekers are at serious risk in both Hillside Haus and West Lorengau Haus in the event of another fire."
Read the UNHCR Manus Island Fact Sheet here.