There have been 67 notified cases of Hepatitis C at Canberra's jail since it opened in 2009, new figures show, but health experts said the number is likely to be much higher.
ACT Health figures obtained by the ABC show 16 of those 67 cases were diagnosed in the past year.
New prisoners in at the Alexander Maconochie Centre (AMC) in Canberra are only offered voluntary screening for blood contact diseases, and president of Hepatitis ACT, Melanie Walker, said that meant the Government's figures were conservative.
"Given that it's a voluntary testing regime you would expect there to be more," Ms Walker said.
"Rates of Hepatitis C are higher in those people who are detainees across the country at any given time, and that also holds true for the ACT."
In 11 of the 67 Canberra Hepatitis C cases, the prisoner caught the virus in jail, the figures show.
ACT Health said 12 cases were identified as in-community transmissions, 28 cases could not be determined with certainty due to the person being in and out of custody, and 16 cases were still being assessed.
Ms Walker said there were likely to be more in-jail transmissions, due to needle sharing and other risky behaviour.
"You would have to say that looking at the probability there would have to be over 20 people who've caught hepatitis C in the prison environment in the ACT, and that's a problem for everyone, I think," she said.
This week, a health summit in Melbourne was told that a third of Australian prisoners have Hepatitis C but less than 1 per cent were receiving treatment.
Only three detainees at Canberra's AMC were receiving treatment for Hepatitis C, with 6 more being assessed for entering a treatment program.
Each program runs for 24 weeks with 10 places available at one time.
Since 2009, 85 people at the jail have commenced treatment for Hepatitis C infection, with 43 completing their treatment while in custody and others withdrawing from treatment due to side effects.