Thousands of boatpeople have been brought to the small Indian Ocean island this year, forcing some to sleep in tents and spreading resources more thinly, the Australian Human Rights Commission said.
"The substantial increase in the number of people in detention has led to overcrowding," the commission said in its annual report on the facility, which lies some 2,600 kilometres (1,610 miles) northwest of Perth.
"There has been a significant deterioration in living conditions for many people, particularly those accommodated in tents and dormitory bedrooms.
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The commission said while improvements had been made since 2009, some inmates were harming themselves, mental health services were limited and children were still being kept on the island.
It said the government's decision to suspend processing claims from Sri Lankan and Afghan asylum seekers for some months in 2010 had led to the prolonged detention of many people, including children.
"More people are being held in immig