State-owned enterprises partly sold and listed on the stock exchange under the National Party's mixed ownership plans will no longer be subject to the Official Information Act.

They would be exempt in the same way as 75 per cent taxpayer-owned but NZX-listed Air New Zealand.

Up to 49 per cent of Mighty River Power, Meridian Energy, Genesis Energy and Solid Energy would be sold over several years if National wins November's election.

More of Air NZ would also be sold.

One senior Meridian manager told the Business Herald exemption from the OIA would be welcomed by SOEs, which found many requests were time-consuming and exposed them to commercial risk, resulting in many details being withheld.

An energy campaigner, Molly Melhuish, said the OIA was "extremely important" in tracking SOEs, particularly on issues surrounding safety.

In 2008 the Government required SOEs to report more financial information more frequently as part of an efficiency drive, although with some exceptions this often falls short of their listed peers.

It also required annual public meetings, which have been low-key, tame affairs, apart from the first one when Solid Energy chief executive Don Elder had a custard pie thrown in his face by environmentalists.