The former head of the Australian Border Force has said all deaths within Australia’s offshore immigration regime should be investigated by an Australian judge or coroner.

Roman Quaedvlieg said the current system – where deaths are rarely formally investigated – led to speculation and criticism, and a failure to address systemic problems.

“As it currently stands, the facts of a death are held across participating jurisdictions and across multiple agencies and individuals, and this fragmentation of information results in uninformed speculation, criticism, and debate,” he told Guardian Australia.

“My experience has been that coroners’ offices, except for the occasional anomaly, are not generally activists and could be relied upon to undertake forensically based inquiries to determine cause of death and produce recommendations to avoid or minimise future deaths.”

Since 2014, 12 people have died in offshore detention – through murder, medical neglect, misadventure and suicide – but only one death has been brought before a coroner.

Hamid Kehazaei died in 2014 after a series of cascading “multiple errors” and “systemic failures” in the healthcare system inside the Manus Island detention led to his death, from a simple, treatable infection that degenerated through poor treatment to sepsis, cardiac arrests, and ultimately brain damage and organ failure.

In a scathing 140-page findings report, Queensland coroner Terry Ryan highlighted that the medical clinic did not have basic antibiotics to treat common tropical infections, that non-medically trained bureaucrats in Canberra ignored emails for up to 13 hours, that those bureaucrats rejected doctors’ recommendations Kehazaei be moved to a hospital, and that Kehazaei was ultimately moved to a hospital that did not have capacity to treat him, and where he was left unattended as he grew critically ill.

But Kehazaei’s death was only investigated because he died in Queensland, and technically “in custody”, which mandates a coronial inquiry.

Fazel Chegeni Nejad’s 2015 death on Christmas Island has also been referred to the coroner, and an inquest is currently being heard in Western Australia. Despite being a remote island, detention on Christmas Island is classified by the Department of Home Affairs as being “onshore” because it is Australian territory.

Ryan said given Australia’s control of and responsibility for offshore processing, all deaths within the context of offshore processing should be mandatorily referred to an “independent judicial investigation” under the auspices of the Australian attorney general.

Quaedvlieg said where an asylum seeker or refugee had died in or around an offshore processing centre, a statutory process, including a coronial inquest where appropriate, would be a useful addition to the current disparate inquiries across jurisdictions.

“This would ensure that an independent, fact-based inquiry is undertaken and a reliable report is produced, enabling more informed commentary, decision-making and, where necessary, adjustments to policy and practices,” Quaedvlieg said.

Quaedvlieg, a former Queensland policeman, was the first commissioner of the ABF but his appointment was terminated last year for alleged misbehaviour and is reportedly the subject of an ongoing corruption investigation. Quaedvlieg has denied the allegations.

He said the countries to where Australia sent boat-borne asylum seekers for offshore processing – Nauru and Papua New Guinea – had limited capacity to undertake coronial inquiries.

“It would be reasonable for those inquiries to occur in Australian jurisdictions if the parties agreed,” Quaedvlieg said.

A spokesman for the Attorney General’s Department said it would “consider the coroner’s report in detail including all recommendations relevant to the attorney’s portfolio”.