The Corrections Department says it expects cold and darkness to force two prisoners out of a disused guard tower at Paremoremo prison, near Auckland.
The inmates scaled the tower during an exercise break in the prison yard on Friday morning, and are refusing to come down.
General manager of prison services Jeanette Burns says the men have not made any demands yet, and it is not clear why they chose to climb the tower.
She says there is no shelter or comfort where they are, and prison staff will not provide food and water until they come down.
She says usually in these situations prisoners come down of their own accord.