The Dunedin Prison Trust raised about $600,000 for the first stage of the building development programme returning the front of the building to its original appearance.
It now planned to restore all three cellblock roofs, which have deteriorated more quickly than anticipated.
Trust chairman Owen Graham said the roof of the 120-year-old building needed to be repaired to guarantee the condition of the category one historic building.
Sections of the roof's structure would also be repaired when necessary, Mr Graham said.Early estimates put the cost between $400,000 and $550,000.
Roof slates would be sourced ether from Spain or the United Kingdom, at a cost of at least $50,000, and then stored until they were needed.
There was no danger to the building or people inside but Mr Graham said work should start by the middle of next year.
Tours of the building would start again after Labour Weekend.
The single tenant, The Escape Room, would remain open during the work.
