Detaining people seeking asylum in Australia more than doubles their odds of developing post-traumatic stress disorder.

A national study published in the Journal of Traumatic Stress investigated the mental health impacts of immigration detention on 334 asylum seekers.

Australian researchers Walter Forrest and Zachary Steel drew on results of a long-term study by commonwealth officials, external stakeholders and refugee experts.

“We conservatively estimate that detention more than doubled the odds of having probable PTSD and likely increased the odds of developing probable PTSD more than seven times,” the authors said in their paper.

“These estimates are extraordinary considering the reported prevalence of PTSD among resettled refugees in other settings.

“Even if most detainees recover from the distress of detention, its consequences may endure if the distress occurs during critical periods.”

These periods included entering the workforce, enrolling in education and caring for young children.

The researchers measured mental distress using the Kessler-6 Psychological Distress Scale, which asks participants to rank emotional states ranging from despair to anxiety, as well as the PTSD-8 screening tool from the Harvard Trauma Questionnaire.

The authors considered several indicators of traumatic pre-migration experiences such as being exposed to war, violence and natural disasters.

They argued long spells in detention may coincide with more severe distress.

PTSD can also impact physical health by exacerbating breathing and digestion issues and intensifying incidents of pain.

The authors restricted their analysis to asylum seekers on the Australian mainland who were listed as the primary applicant on their visa forms.

Some 234 people came to Australia by boat and were classified as unauthorised maritime arrivals, while the remaining 100 asylum seekers arrived in Australia by other means.

Participants came from Afghanistan, Iraq, Iran and Pakistan.

Under the Migration Act, non-citizens who enter Australia without a valid visa must be detained.

Two-thirds of the participants surveyed had spent time in immigration detention before they were granted protection.

Most participants spent less than five months in immigration detention, well below the average length of more than two years.

The latest government figures show more than 1100 people in immigration detention facilities.

Last week, the government brought the last asylum seekers from offshore detention on Nauru to Australia.

The empty centre will cost Australia $350 million per year to maintain.

The Department of Home Affairs said its policies towards asylum seekers who arrived by boat had not changed – they would be processed in offshore facilities and given no chance to resettle in Australia.

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– AAP