Hiring private firms to partially sell four state-owned energy companies is going to put a $100 million hole in the government's books, the Greens say.

Treasury has announced it's appointing a panel of firms to sell 49 per cent of the shares in Mighty River Power, Meridian Energy, Genesis Energy and Solid Energy.

It expects as many as 250,000 investor applications and fees charged by brokers will run to about $100m.

Greens co-leader Russel Norman says it's taxpayer money and will be a windfall for banks and finance companies.

"The amount set aside for paying companies to market this sale is huge," he said.

"This ideologically-driven policy will put a $100m hole in the Crown's books, according to the government's own numbers - it's selling selling state-owned assets out from under us and making the taxpayer pay for the privilege."

The government expects to raise about $6 billion from the share sale, which opposition parties strongly oppose.