None of the prisoners are yet to be confirmed as having COVID-19 but some coronavirus tests have been administered.

"Any inmate who develops a fever or respiratory symptoms is immediately isolated from the general prison population and provided with a surgical mask," a Corrective Services NSW spokeswoman told AAP on Thursday.

"Staff from Justice Health and Forensic Mental Health Network will assess their symptoms and undertake further screening, such as whether the inmate has a history of respiratory illness."

COVID-19 test results are returned within three days, according to Corrective Services NSW.

"If the test result is positive, the inmate will receive appropriate medical care from Justice Health."

As a precaution, all inmates who develop coronavirus-like symptoms will be put into isolation.

The development came as Civil Liberties Australia added its voice to a growing call for prison populations to be reduced.

"Prisons and other places of detention are a serious challenge to social distancing measures and therefore represent a threat to the health of both the prison population and to the wider community," the organisation said in a statement on Thursday.

"State and territory governments must adopt measures as a priority to minimise prison populations and reduce crowding in these facilities."

The NSW government passed laws last week giving the corrections commissioner the power to grant conditional parole to low-risk offenders where necessary.

While the authority to act now exists, AAP understands at this stage there are no plans for the early release of prisoners.

Australian Associated Press