Rio Tinto is being warned it can't sell off assets in New Zealand for two more years.

The group, a British-Australian multinational metals and mining corporation, has headquarters in London and Australia. The company is working on selling some Australasian aluminium assets, including Tiwai Point near Bluff.

According to Britain's Financial Times newspaper, Rio Tinto has hired Credit Suisse to find a buyer for Pacific Aluminium, majority owner of the Tiwai Point smelter at Bluff and owner of a number of smelters in Australia.

However, because the company got a $30 million cash injection from the government in 2013, finance minister Bill English says the company cannot sell off Tiwai Point just yet.

"There's a commitment there through to 2017 and beyond that, it's their business."

Pacific Aluminium owns 80 percent of the Tiwai Point smelter at Bluff and Japan's Sumitomo Chemical holds the remaining stake.

The future of the plant has been uncertain in recent years, and English believes the government won't have any further financial part to play.

"That was discussed last time around. We were very very clear with Rio Tinto that there'd be no further contributions."