Just over a third of Victorian prisoners are fully vaccinated against COVID-19, despite being a priority in the rollout.
The Victorian Department of Justice has confirmed 37 per cent of adult prisoners have been fully vaccinated, while 55 per cent have received their first dose.
Forty-five per cent of staff are fully vaccinated, while 71 per cent have received their first dose.
All staff and prisoners across adult custodial facilities were included in Phase 1b of the national vaccine rollout, which began in late March.
On-site vaccinations at Victoria's prisons did not begin until June.
"There are currently no confirmed cases of COVID-19 in the Victorian prison system," a department spokeswoman told AAP on Wednesday.
"Vaccines will be made available to prisoners for as long as they are needed via the Department of Justice and Community Safety's on-site vaccination program."
Prison populations are not static, with unvaccinated inmates entering Victoria's prisons every day.
All new prisoners are tested and required to spend 14 days in protective quarantine, regardless of COVID-19 risk.
It comes as all NSW prisons have been locked down after an outbreak at the privately-run Parklea jail more than doubled to 75 cases on Tuesday.
The NSW government has refused to reveal the level of vaccination among both prisoners and staff.