He also says prisoners are put on methadone when they have never had a heroin habit, creating an addiction chain that follows them on the outside.

His claims about methadone are given weight by new figures from the Australian Institute of Health and Welfare showing 27 per cent of ACT prisoners are on methadone, a rate that outstrips any other jurisdiction by a long way. In Victoria and NSW, the rate is 10-11 per cent, in South Australia 7 per cent and elsewhere it is negligible to zero.

Witness J says visitors are the main source of drugs in the prison, with drugs handed over when visitors and prisoners kiss, or in babies' nappies - with soiled nappies used to stymie drug-detection dogs. Children are also made to carry drugs past the security checks.

Drugs are also secreted into books donated to the prison library. They are kicked over the fence in "drug-laden footballs" and rarely drones have been used, he says.

One prisoner claimed to have paid upwards of $5000 for drugs, "and I'm surprised to learn there is a corrections officer in a part of the process," Witness J writes in his memoir about his 15 months in Canberra's jail. The prisoner, who was released in August 2019, can't be named for national security reasons.

Mobile phones are banned at the prison, creating an incentive for an illegal trade. Witness J said he himself had two phones in jail, both cheap Nokia 1 phones with a dongle so he could access wifi, which he bought via a corrupt guard and for which he paid $1000 and more than $1000. He says the money is paid to visitors or into the bank accounts of friends or relatives of the guard.

Most guards are "a decent bunch of people doing a tremendously monotonous job", but some can be bribed to bring in a mobile phone, drugs or alcohol.

In an interview, Witness J, who is banned from revealing his true identity and spent 15 months in jail under a pseudonym, said there were "some brilliant, brilliant human beings working as correctional officers absolutely, it's not rotten structures". But there were "a few rotten apples".

"Certain guards are complicit in the introduction of illicit items around the jail; as I've perhaps mentioned, it's quite common knowledge that one particular guard will get you a mobile phone for $1000 but the brazen nature of this activity causes my eyebrows to rise," he writes in his memoir.

"I'm simply amazed that this is allowed to happen and wonder how much the senior management knows about and accepts these things, as any serious effort to have these corrupt guards fired surely cannot take much work from an intelligent and focused person with the right access."

A Justice and Community Safety spokesperson said authorities took all allegations of staff corruption very seriously but "to date, no allegations of staff members engaging in the introduction of mobile phones or illicit substances for the purpose of selling to detainees have been substantiated".

Allegations of corrupt or criminal conduct were referred to ACT Policing, the ACT Integrity Commission or the ACT Public Sector Standards Commissioner for investigation.

"The former detainee is encouraged to report any information they may have relating to these allegations to Crime Stoppers for investigation," the spokesperson said.

Witness J also revealed the extent of methadone use in the prison, saying it is used to treat pain, with prisoners telling him they're not allowed stronger painkillers than panadol - so they're put on methadone instead.

Serial burglar and rapist Shaun Burke was on methadone in jail for crippling pain from three bulging discs, he says.

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"I'm astounded that a health system that seems to put so much effort into avoiding patient addiction would slap someone on to methadone for pain relief," Witness J comments.

"It's perhaps the cruellest irony of all: prisons that put detainees on to methadone, sometimes for years, and then release them into the clutches of heroin."

The ACT Human Rights Commission has also raised concern about methadone rates and Indigenous health leader Julie Tongs has described the practice as "liquid handcuffs.

But a Justice health spokesperson said all inmates on the prison's opioid maintenance program had been diagnosed as opioid dependent - either on heroin or prescription opioids.

While the Institute of Health and Welfare data shows 127 inmates on methadone and five on buprenorphine in 2019, the spokesperson said in April this year, 50 inmates were on methadone, with 56 on buprenorphine and eight on suboxone. Other states use buprenorphine in small numbers.

While some other jurisdictions did not offer methadone in prison and others capped the number of prisoners, the ACT did not cap numbers on opioid maintenance treatment, the spokesperson said. Methadone was rarely used for pain management and no-one was on methadone for pain at the moment.

Witness J says the attitude to contraband ranged from "sensible to ridiculous". Sugar is out because it can be used to make alcohol and for the same reason they are not allowed to grow potatoes but they can buy potatoes for cooking. Chilli and pepper is banned because it can be weaponised. But "drugs, syringes and tattoo guns seem to flow unimpeded through the jail".

Most prisoners must eat with plastic cutlery from cardboard containers. The cottage where he was houses has a large butcher's knife, chained to the counter, and other kitchen equipment, but plastic cutlery.