The Commonwealth Ombudsman has launched an investigation into allegations several officers harmed a detainee, who was handcuffed behind his back and held down on his stomach for 45 minutes. The Turkish national said he struggled to breathe and was denied repeated requests for water. Serco also confirmed it has sacked two officers over an assault on a Sri Lankan detainee, who was pushed and punched in the face in December. Another reported attack, investigated internally, involved a middle-aged Chinese woman allegedly being kicked in the stomach by a guard. The incidents are the latest to spark scrutiny of Serco, which runs several detention centres in Australia and offshore. The company has previously come under fire over its handling of riots, overcrowding and suicide attempts at the Christmas Island detention centre. Dozens of complaints have been lodged by inmates at the Maribyrnong detention centre in recent years, mostly about staff harassment and bullying. But insiders say the recent allegations of brutality shows the centre was being run like a "prison camp". Serco officers based at Maribyrnong have told Fairfax Media a more aggressive attitude towards detainees has filtered down in recent from new centre managers, and there had been "repeated assaults on detainees".

"We have got detainees who are being literally bashed … viciously assaulted," one officer said. "If this sort of thing happened six or 12 months ago people would not only be sacked but even charged with an offence. Another said senior staff condoned the use of excessive force against detainees. "Managers are quite happy to use handcuffs behind people's backs and push them down on their stomachs. Even under the Howard government, we had a lot of power but we didn't use this kind of force." Pamela Curr, of the Asylum Seeker Resource Centre, said the Maribyrnong centre had begun housing more detainees whose visas had been cancelled alongside asylum seekers. She said Serco had transferred a number of "hard-line" managers out of the prison system into the centre late last year and had adopted a more aggressive approach over concerns about smuggled drugs. "Ex-prison guards have a very different culture and mentality to officers who have been trained to guard asylum seekers," she said. "People inside Maribyrnong are saying it is treated like a prison and is a very hard place to be."

An Immigration Department spokeswoman said it investigated all claims of staff misconduct and was co-operating with the ombudsman's probe into allegations excessive force was used against a detainee. "The department expects Serco to take suitable disciplinary action against staff who have been proven to have acted inappropriately," she said. "The department takes its duty of care to detainees very seriously and works closely with Serco to ensure standards are met." Serco said in a statement its own investigations had found "no evidence ... to substantiate allegations of misconduct" in two incidents, but a third inquiry resulted in dismissals. "Our officers are trained to de-escalate incidents where individuals become aggressive, and only to use approved control and restraint techniques as a last resort," the statement said. Liberty Victoria president George Georgiou, SC, said force used against detainees must be reasonable and proportionate.

"I can only see those circumstances being where an officer needs to defend themselves, another officer, or to protect another person," he said. "I couldn't see that there would ever be a situation where someone needs to be handcuffed for a prolonged period, where someone would need to be kicked." Mr Georgiou raised concerns about the process of internal investigations and said people who made complaints in detention should be kept informed. "Many bodies do conduct internal investigations but where alleged criminal offences are involved or where a person's human rights are involved there should be an independent investigation. That process should be open, it should not be done in secret."