New Zealand's largest farm, Molesworth Station, officially becomes a protected park tomorrow preventing the National Party from ever selling it, Conservation Minister Chris Carter said today.

"Molesworth extends across 180,000 hectares of South Marlborough high country, an area full of threatened native species and superb tramping and mountain biking opportunities. From tomorrow (July 1) Molesworth will be formally managed as a protected farm park by the Department of Conservation," Mr Carter said.

"Until now, Molesworth was managed by Land Information New Zealand in conjunction with Landcorp, the government's land and farming agencies. Because of this, the station was vulnerable to sale if a National government were to ever get elected again. By protecting the station, and transferring its management to DOC, the Labour-Progressive government has guaranteed a magnificent part of New Zealand remains in public ownership.

"The threat to Molesworth was real. National has recently announced its desire to close down Landcorp and sell off its 112 farms, which could easily have included Molesworth Station if we hadn't acted," Mr Carter said.

"Molesworth is now safe, but another 170,000 hectares of New Zealand land currently owned by the public through Landcorp are not. Overseas investors are salivating at what may prove to be the biggest fire sale of Kiwi land ever if National is one day elected to government."

Mr Carter said while DOC would now manage Molesworth Station as a reserve, Landcorp would continue the historic farming operation on the station under a new lease arrangement.

"DOC will spend an additional $2.7m on the station over the next five years, enhancing it as a visitor destination, and protecting its natural and historic heritage. Access to the station for recreation is being increased, and a new east-west tramping route is to be developed," Mr Carter said.