Australia's government spent more than US$1.5 billion on its offshore detention centres for asylum seekers without proper authorisation and mishandled its contracts, an audit has found.
In a report, the National Audit Office said the immigration department had "fallen well short" of expected standards in its management of contracts for the centres in Nauru and on Papua New Guinea's Manus Island.
It detailed "significant shortcomings" in contracts for security and welfare at the centres, and did not get "value for money" when establishing Australia's detention regime.
Of the US$1.7 billion paid over three years, US$830 million was approved by officers without the appropriate authorisation and another US$830 million was paid with no record of who had authorised the payments.
The department also failed to update its asset register to take into account a new $US56 million facility in Nauru - which meant is was not insured when it burnt down in a riot in 2013.
Since 2012, any asylum seeker who tries to reach Australia by boat is sent to the centres for processing, before being resettled in another country.
But processing has been slow, with few refugees resettled, and the system has been riven by allegations of systemic human rights abuses, including physical and sexual abuse by security guards.
The report also raised several safety concerns, including gaps in video surveillance at the centres and issues with mould.
In a statement, the immigration department denied it made payments without proper authorisation, or that it failed to give due consideration to value for money.
But it acknowledged a lack of documentary evidence and agreed to implement the office's recommendations, including to take "immediate steps" to strengthen procedures and comply with its own accountability instructions.