The Corrections Department is refusing to reveal which of its 18 prisons pose the greatest risk in an earthquake.
The department will not release a seismic risk assessment under the Official Information Act, saying the document is commercially sensitive.
Opposition parties say the secrecy is extraordinary and the public interest outweighs any commercial sensitivity.
The Labour Party said on Thursday there is no justification for the secrecy, which it says does more harm than good.
The Green Party suspects that the report may contain something embarrassing.
Corrections Minister Anne Tolley said she has not seen the report, but would expect to have been told if any significant problems were uncovered.