Figures also show the territory and NSW have the equal highest use of all pharmacotherapy treatment - which replaces opioid drugs like heroin with a legally prescribed substance - across the broader community at a rate of 26 per 10,000 people.

ACT Health said the higher rates of opioid replacement therapy, which include methadone, point to better access to treatment.

Alcohol Tobacco and Other Drug Association ACT (ATODA) said methadone was an important treatment for prisoners, but drug treatment across ACT's health and justice system needed significant improvement.

Data from a snapshot day in June 2016 shows 128 ACT prisoners were prescribed methadone.

Prisoner numbers, also taken on a snapshot day in June 2016, show there were 441 prisoners in the ACT, meaning 29 per cent of all prisoners were prescribed methadone.

The percentage of prisoners prescribed methadone in other jurisdictions was significantly lower, the second highest being Victoria and NSW both about 12 per cent of prisoners.

Tasmania, Northern Territory and Queensland had close to zero per cent prisoners prescribed methadone.

Methadone reduces withdrawal symptoms in people addicted to heroin or other narcotic drugs.

While it is an accepted and widely practiced treatment, it can be dangerous if prescribed to someone who does not have a legitimate addiction.

Justice health service made changes to the prescribing of methadone in ACT prisons after the death of Steven Freeman in 2016 who started on the methadone program two days before his death.

Changes since his death have included a new electronic methadone dosing system for inmates, to replace the existing manual system, and reducing the initial starting dose for inmates from 30mg to 20mg.

Despite the changes, a long awaited review into the ACT's guidelines governing opioid maintenance treatment, which began in 2012, is still yet to be completed, despite health minister Meegan Fitzharris earlier this year saying it would be completed within six weeks.

An ACT Health spokeswoman said the number of AMC detainees on the opioid replacement treatment program had decreased since 2016.

However it still accounts for more than a quarter of the prison population.

In November 2016 141 of the 445 AMC population was on the opioid replacement, dropping to 120 of 457 detainees as of November 22 this years.

"It's important to emphasise that higher treatment rates should be viewed as an indication of access to treatment for this client cohort, rather than as a measure of an underlying drug problem in the ACT community," the spokeswoman said.

The spokeswoman said ACT Health was working "with a view" to finalising the outdated guidelines by the end of the year.

"For the last three months ACT Health has and will continue to consult with relevant stakeholders and service providers with a view to finalising the guidelines before the end of the year," she said.

"The review will not result in significant changes to the delivery of local programs or procedures.

"The review proposes adoption of the national guidelines for medication-assisted treatment of opioid dependence 2014 under ACT legislation, to ensure ACT guidelines are nationally consistent and reflect evidence based best practice."

ATODA's CEO Carrie Fowlie said opioid maintenance therapy is the most common, effective and cost-effective treatment for heroin and opioid dependence in Australia and in the ACT.

She said it is associated with lower rates of offending and reduces risk of death upon release for opioid dependent people.

"ATODA has long advocated for more coherent planning, funding and delivery of drug treatment across the ACT's health and justice systems," she said.

"Significant improvements are needed, for example ACT's primary drug policy governance and monitoring mechanism, the ACT Alcohol Tobacco and Other Drug Strategy Evaluation Group, has not been convened in 2017."