Mighty River Power directors working to get the state-owned company ready for a partial sell-down were paid an extra $24,000 each - totalling $220,000 - for about 20 days' extra work.

And the government has indicated their fees could rise after the sale.

The government on Tuesday passed legislation enabling it to sell up to 49 per cent of four state-owned energy companies - Mighty River Power, Genesis, Solid Energy and Meridian - and Air New Zealand.

Mighty River Power will be the first to go on the block later this year.

The boards of the four energy companies were given $120,000 to start preparing the companies for share offers from last July, the New Zealand Herald reports.

When Mighty River was chosen as the first to be sold, its board chairwoman Joan Withers asked State Owned Enterprises Minister Tony Ryall for extra money to get the company ready.

She said up to 190 days' extra work, or about 21 days' work for each of the nine directors - who are paid $1200 a day - to a maximum of $228,000 was needed.

State-Owned Enterprises Minister Tony Mr Ryall wrote in a briefing paper that the payments are "a lot of money" and strong record-keeping is needed to ensure it is well-spent.

He also said the $1200-a-day rate for directors was being reviewed because of the "additional risk and responsibility of publicly listed companies".

Prime Minister John Key on Tuesday said he was not concerned that directors' fees would increase when the company shares were floated.

"They reflect that there are greater responsibilities on the director of a (private) company and they're everything from continuous disclosure to much more personal liabilities, so at the end of the day I'm not terribly surprised."