The Northern Immigration Detention Centre in Darwin is the worst in Australia, according to the Greens.
They say there have been almost 100 incidents of self-harm or threatened self-harm at the centre in the last year.
Greens Senator Sarah Hanson-Young has told the ABC’s Four Corners program that when she visited the centre this year, it reeked of depression.
“(They are) almost shadows of people, hunched over,” she said.
“When they look at you, (their) eyes are all glazed and red. They are obviously on a lot of anti-depression medication, suffering huge mental health issues.”
A mental health nurse who has worked at the Darwin detention centre told Four Corners she had been trying to stop detainees from harming themselves on a daily basis.
The nurse, whose identity has not been revealed, says up to 30 detainees could be on suicide watch at the same time.
She says people were more at risk the longer they were held at the centre.
“By six months you usually had a reaction and they would start to have difficulty sleeping,” she said.
“They would be having thoughts of wanting to harm themselves.
“By nine months they had probably acted out on those things, and those thoughts were coming more and more.”
The Australian Medical Association says up to half of children being held in immigration detention in Australia could be suffering from post-traumatic stress disorder.
The AMA says the policy of mandatory detention is medically harmful and violates basic human rights.
It says the detention of children is a form of child abuse.
AMA spokesman Peter Morris has told Four Corners that studies have already been done to measure mental illness among children in detention.
“Recent research and systematic reviews of the research from many countries, involving many thousands of children, have found overall between 10 and 30 per cent of children are suffering from depression, and between 20 and 50 per cent have post-traumatic stress disorder,” he said.
Federal Immigration Minister Chris Bowen says the government is taking steps to move people out of detention.
“We have moved a lot of people into the community over the last 12 months, including adults who have suffered torture and trauma, and who are particularly vulnerable,” he said.
“We will use bridging visas more regularly to move more people into the community because I do think that is appropriate.”
The Darwin detention centre has a nominal capacity of more than 350 people.
A new 1500-bed detention centre is now being built at Wickham Point, about 35 kilometres south-east of Darwin.