She said between 150 and 200 jobs may go out of the 1400-strong workforce.
Stephens said all options had been considered during the review but it needed to take action to ensure the survival of steelmaking in this country in the face of challenging conditions.
"Unfavourable public policy in relation to carbon, energy costs and trade remedies have compounded the financial pressures and the business can no longer continue in its current operating model."
The mill made an operating loss of $6.5 million (AU$6m) in the past year, and Bluescope had taken a $213.42m (AU$197m) writedown of the business in its latest results.
The business includes the steelmaking plant at Glenbrook, south of Auckland, and Pacific Steel, and it had already closed down one production unit last year.
Bluescope said last month the restructuring was expected to cost between $32-54m (Au$30-50m).
- RNZ