Official papers show increasing sentences for the most serious driving offences will cost the Corrections Department about $12 million.
The papers, which were obtained by Radio New Zealand under the Official Information Act, also show the Cabinet went against advice from the Ministry of Transport when it deferred a decision on whether or not to lower the adult blood alcohol level.
In July, Transport Minister Steven Joyce announced maximum sentences for drunk, drugged, dangerous or reckless drivers who cause death, would be doubled to 10 years in prison.
The papers shows the changes are expected to lead to an estimated increase of 25 prison beds, at a capital cost of about $10 million.
Annual operating costs to cover the additional prisoners are expected to be more than $2 million.