Accounts of the fatal violence on Manus Island took a more sinister turn on Tuesday, as it emerged that G4S guards allegedly opened the doors of the camp to a local dog squad and PNG police, hours before the riots erupted and in the knowledge of threats against detainees by local maintenance staff. The new eyewitness account bolsters allegations that G4S is directly or indirectly responsible for the bloodbath that ensued.

The private security firm’s role in the riots, though shocking, comes as no surprise to anyone familiar with the history of abuse allegations that have been levelled at G4S and Serco in their detention operations abroad.

In the past six months alone, G4S and Serco have been embroiled in allegations of outrageous proportion.

In October 2013, South African authorities assumed control of a G4S-run prison, after it descended into violent chaos amid allegations of torture of inmates by G4S guards. Among the allegations are that G4S guards forcibly injected inmates with anti-psychotic drugs, even though they were not suffering from psychosis, and administered electric shocks for purely disciplinary reasons. G4S denies all allegations of abuse, but a G4S employee said guards used electric shields because they were “hopelessly outnumbered by dangerous prisoners.”

The prison broke out into riots and, on three occasions, prisoners took G4S guards hostage. South African authorities intervened after finding the firm had “lost effective control” of the prison.

Serco’s most recent scandals in the United Kingdom have included allegations of sexual abuse in Yarl’s Wood immigration removal centre, where detainees await deportation from the UK. Detainees have alleged sexual and indecent assault, and identified systemic patterns of sexual abuse of vulnerable detainees who are led to believe that succumbing to guards’ demands for sexual favours represents their only hope of being released from detention.

Manus Island riot footage on 16 January. Photograph: ABC Photograph: ABC

Serco denies the allegations of abuse, and the only disciplinary action has been to dismiss an employee for “inappropriate behaviour with a resident”, (closer to home, and only a couple of months after the Yarl’s Wood dismissal, a Serco guard on Christmas Island was sacked for entering into a sexual relationship with a detainee.)

More shocking still is Australia’s apparent indifference to allegations against its contractors. The fact is, each time allegations surface, Australia is put on notice about the risk of similar allegations occurring in its own offshore immigration detention centres. Given the track records of G4S and Serco, the public should demand to know how the Australian government could possibly be satisfied of the firms’ competence to ensure the safety and welfare of detainees in their care.

During Question Time on Monday, Scott Morrison insisted that Papua New Guinea was ultimately in control of the Manus Island detention centre. The implication is clear: the Australian government is attempting to distance itself from G4S and from the reports of what occurred there. But the fact is that G4S is on Manus Island thanks to a contract with the Australian government. It is being paid with Australian taxpayer dollars, in furtherance of an Australian government policy.

Transfield’s impending takeover of the G4S contract on Manus Island does little to assuage concerns about detainee welfare. Transfield comes with a different track record problem, in that it barely has one. Transfield is starting off on the wrong foot if it thinks debate about the administration of offshore immigration detention is no different from controversy over its role in major infrastructure projects such as the Sydney Harbour Tunnel. Ensuring the welfare of vulnerable and traumatised people is not like building a road.

If nothing else, the abuse allegations against contractors demonstrate that outsourcing immigration detention requires an abundance of oversight, not a cloak of secrecy.

Australia should, like South Africa, assume public control of its privatised detention centres until it can absolutely guarantee the safety and welfare of detainees. And it should, like the United Kingdom, place an embargo on Serco and G4S from signing any new contracts while it undertakes a full, public investigation of all allegations of contractor abuse.

The very least we can do is end outsourcing of detention operations and assume public control of the camps that hold some of the most vulnerable people in the world. In failing to control its contractors on Manus Island, it might be that the Australian government does indeed have blood on its hands.