Australia "cannot avoid responsibility by contracting out their obligations to other states, international organisations or private entities," the report, written by the UN's special rapporteur on the human rights of migrants, said.
It has been the policy of successive governments to send undocumented asylum seekers offshore for processing, or hold them indefinitely in domestic immigration facilities.
Australia signed a $408 million deal with Nauru in August, under which the Pacific nation is resettling more than 350 non-citizens.
The cohort had been held in indefinite immigration in Australia before the High Court found the practice was illegal given they had no reasonable prospects of deportation, in a landmark 2023 ruling.
Several members of the group had a history of violent offending.
"This report confirms what we have long said, that Australia remains responsible for the harm caused by offshore detention and is yet again more condemnation of a cruel system," deputy chief executive of the Asylum Seeker Resource Centre, Jane Favero said.
"The Albanese government cannot wash its hands of the people it has sent to Nauru by paying another government or private contractors to carry out its policies"
People transferred to Nauru since the arrangement have reported sexual abuse, including against children, and isolation.
Many were also at risk of being separated from their family after having lived in Australia for years, the report said, while pointing to similar trends across the world.
It also called for more transparency around offshore processing and agreements with foreign governments, which have, in the past, rarely been made public.
"Over the past decade, and continuing today, it is only through whistleblowers, advocates, lawyers and refugees that the harm and abuses offshore have been exposed," Ms Favero said.
"This UN report makes clear that transparency and accountability about offshore detention is not optional."
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