A report by the Commonwealth Ombudsman released on Wednesday revealed 30 Australians were wrongfully held in immigration detention by the Department of Home Affairs in 2024/25.
It marked the largest spike in cases linked to communication failures since 2005.
"Wrongfully depriving a person of their liberty is serious," Commonwealth Ombudsman Iain Anderson said on Wednesday.
"Since we began monitoring the issue in 2005, we have observed the same types of errors causing people to be wrongfully detained."
Most wrongful detention cases were the result of errors previously known to the department, and could have been avoided if existing policies and procedures were properly followed.
One case included an Australian citizen held for two and a half years before being released.
The case, referred to as Mx B, involved an Australian citizen who was unlawfully detained for 945 days.
During their detention from 2022 to 2025, Mx B claimed to be a citizen on multiple occasions, but three checks by the department failed to show evidence.
A fourth assessment confirmed Mx B was an Australian.
"In April 2025, the last citizenship status assessment found they were a citizen as they had acquired citizenship on their 10th birthday by operation of law," the ombudsman's Righting Wrongful Detention report said.
"However, this finding only occurred after Mx B was released from immigration detention."
An internal review into the matter was launched in June 2025, with a report on the outcome expected once available.
The ombudsman had yet to determine the status of the review as of January 2026.
"As of the publication of this report, the review had yet to be finalised," the report said.
"It is concerning that the same systemic errors continue to result in wrongful, and in some cases unlawful, detention of Australians."
In 2023/24, the ombudsman found the issues leading to people's wrongful detention were, in almost all cases, not new.
"Home Affairs has taken steps to strengthen its processes for assessing an individual's immigration status before planned detention and has enhanced the redress pathway for people who have been wrongfully detained," Mr Anderson said.
"The cases examined in this report, however, demonstrate that significant shortcomings remain, specifically regarding the wrongful detention of Australian citizens."
Three recommendations have been made to improve department policies and procedures to limit the risk of wrongful detention, and to provide people with information about the circumstances of their wrongful detention.
Home Affairs has accepted all three recommendations.
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