Minister of Health David Cunliffe says he has no way of making Pharmac fund a 12-month course of the breast cancer drug Herceptin.
The state drug-buying agency on Thursday again declined to fund a 12-month treatment course after a review ordered by the High Court.
Pharmac favours a nine-week course. A year-long course is standard overseas.
Critics say Pharmac's move is "inhumane", politically motivated and flies in the face of international scientific evidence.
But Mr Cunliffe says he is prevented by health and disability legislation from intervening in decisions made by the agency.
He says he has been told by Pharmac that it would not fund Herceptin even if it were given extra money to do so.
Mr Cunliffe says Pharmac has told him there is no evidence a 12-month course of Herceptin is better than nine weeks.