Mighty River Power has reported a $68 million profit after tax, but is refusing to give any insights into future performance ahead of its public flotation.

The 46 per cent fall in profit for the year to June 30 related to accounting charges stemming from falling interest rates.

Earnings before interest, tax, depreciation and amortisation and other charges rose 4 per cent to $461.5m, at the bottom of the company's guidance range.

The company increased its total dividend to the Government by 8.5 per cent over a year earlier to $119.8m, although for the second half, dividends were down 1.5 per cent compared with 2011 at $45m.

Owner of a series of hydro stations on Waikato River, Mighty River has more recently invested hundreds of millions in geothermal development in New Zealand and the United States.

Although a complaint to the Waitangi Tribunal has cast doubt on the process, the Government still wants to sell up to 49 per cent of the state-owned company before the end of the year.

Chief executive Doug Heffernan fended off any questions that could give insight into future earnings, saying he was prevented from doing so by the Financial Markets Authority as it could soon launch an investment prospectus.

This ranged from whether the company believed returns on its retail business were adequate, to how much it was planning to invest in a US geothermal fund which is seeking cash.

Mighty River has invested US$250m (NZ$309.7m) into the GeoGlobal Partners fund, which invests in US geothermal plants.

Mighty River said it had incurred $3.8m in costs related to preparing for a possible public listing, mainly in the form of professional services fees.

Chairwoman Joan Withers said $2.5m of this would be an ongoing cost which would be carried by the company. Asked if preparing it for the rigours of market scrutiny had led to increased efficiency, she had little to say.

- © Fairfax NZ News