Two inmates and 20 Corrections staff at Auckland Prison at Paremoremo are in isolation, after the household contact of a staff member tested positive for Covid-19.
The staff member has tested negative for the virus, and works outside the prison's perimeter.
But Prison Director David Pattinson said as part of a proactive and deliberately cautious approach, all their possible contacts were being asked to take a test and self-isolate.
That also includes eight contractors and professional visitors, including delivery drivers.
The two areas the staff member worked in have been deep cleaned.
No family or friends of inmates have visited the prison since Sunday.