The drop was due mainly to one-off writedowns of overseas geothermal assets totalling $83 million, including $56 million in Chile, and a greater-than-expected drought impact due to lower Waikato River hydro-generation, which declined from $270 million to $258 million - for the six months trading to December.
Electricity sales for the period were up 4%, from $827 million to $856 million.
Because of the ongoing drought, Mighty River has lowered its full-year 2015 outlook for earnings before interest, tax, depreciation, amortisation and changes in the fair value of financial instruments (ebitdaf), down by about $15 million to a range of $480 million to $500 million, from earlier guidance of $495 million to $520 million.
Mighty River will pay a first-half dividend of 5.6c per share, and maintained guidance it intends paying 14c for the full year.
Mighty River shares were down 2.5% at $3.29 following the half-year announcement yesterday.
Craigs Investment Partners broker Peter McIntyre said the result was ''within expectations'', but the decline in the share price yesterday was likely linked to Mighty River's guidance on ebitdaf guidance, which would come in lower than in the previous year.
Aside from struggling with lower hydro flows, there was increased competition in retailing, while separately, Mighty River was reducing commercial sales renewals, Mr McIntyre said.
Forsyth Barr broker Andrew Roone, said while Mighty River was highlighting the dry hydro conditions as being the main cause of the weak result, they had had a greater impact than expected.
''There is a big impairment of the international geothermal business totalling $83 million which, whilst expected, we had not put a specific number on in our forecast,'' he said.
While net profit after tax was down from $124 million to $8 million, normalised net profit after tax (npat) declined from $105 million to $90 million.
Mr Rooney said the revised dividend payout formula and strong operating cash flows, which were up $5 million to $175 million, despite the weaker operating profit result, had allowed Mighty River to pay a higher dividend.
Mr Rooney said because hydro conditions were continuing to be tough, Mighty River had lowered its ebitdaf guidance range, by about $15 million.
''We'll be looking to lower our full-year 2015 forecasts as poor hydrology continues to impact on earnings,'' he said.
However, with the operating cost base reduced, Mr Rooney did not expect there to be a valuation impact as a return to normal hydrology ''should see a strong rebound in earnings''.
Mighty River chief executive Fraser Whineray said hydro generation was constrained by lower rainfall.
During the interim period, inflows to the Waikato River catchment were 18% below average and that had extended into the New Year, with hydrology since December 31 more than 40% below average.
Total generation was up more than 4% on a year ago to 3404 GWh, largely due to using natural gas at Southdown.
