Forty-three prisoners in Parklea Correctional Centre in Sydney's north-west tested positive for coronavirus overnight, bringing the fast-growing cluster to a total of 75.
All of the new infections are inmates, NSW Health has confirmed, with just one staff member among the now 75 positive cases.
Yesterday Parklea Correctional Centre recorded 31 cases.
"The prison is in lockdown and appropriate public health action is occurring," New South Wales Chief Health Officer Kerry Chant said.
Recreation and mingling in the prison yard are off-limits and inmates are isolated in their cells.
Dr Chant said prisons were particularly vulnerable to a Delta outbreak.
"There's vaccination going into the prison," Dr Chant said, explaining that infection control practitioners were now at the prison, trying to stem the cluster's growth.
St Vincent's Health clinical care staff are also assisting infected prisoners.
Parklea Correctional Centre houses maximum and minimum security prisoners.
Six correctional officers at Bathurst Jail in western NSW have also tested positive.
NSW prisons were yesterday plunged into a statewide lockdown, with no visitors allowed.
NSW today recorded 1164 new cases of COVID-19 and three deaths.