Labour and the Greens have proposed a single buyer model for the wholesale electricity market with the crucial aspect being that generators would receive a price for their power based on their individual cost of generation and a "fair" return on capital.
The entire sector had been hit but MRP appeared to be bearing the brunt of negative sentiment, said Mark Lister, head of private wealth research at Craigs Investment Partners.
"Obviously the political issues are the elephant in the room. That's far and away the biggest cloud hanging over the sector [but] Mighty River seemed to be bearing the brunt of the negative sentiment around the generators."
It was also suffering as part of the shift under way from safer, yield stocks to growth stocks.
"People are looking to add more cyclical stocks and the high income stocks are less in favour," he said.
Lister said investors, including his own clients, were fine-tuning their portfolios. The prospect of the remaining state-owned power company, Genesis, coming on the market soon was also affecting the energy company stocks, he said.
About 80,000 new investors bought shares in Mighty River last year and the falling share price would be "disappointing" for them but they would be consoled by the prospect of healthy dividend cheques following the release of the company's half-year results in February.
Mighty River Power's share of electricity sales has dropped compared to a year ago and prices it is getting for power have tumbled.
Operating figures show its share of electricity sales around the country dropped 2 per cent to 18 per cent in the six months to December 31 compared to the same period a year earlier.
In the three months to December 31 Mighty River's average sales price to residential and commercial customers was $111.17/MWh - up just 0.6 per cent - compared with $110.52/MWh in the previous corresponding period.
It has committed not to raise prices to residential customers until at least April 1, next year, Heffernan said.
"It continues to be tough competitively and despite the political rhetoric you're starting to see that with price outcomes for customers.
"They might say their bills are going up but it's not coming from energy, it's coming from other things."
And while MRP says it is outperforming its competitors, the average price achieved for its generation has fallen from $66.25/MWh at the end of 2012 to $47.67/MWh in the final quarter of last year.
Operating figures for the quarter ended December 31 show customer sales volumes were down 9 per cent as the company continued to reduce sales volumes in the low-price, low-margin commercial market.
The company said that in the residential market, seasonal demand was 5 per cent lower on a per customer basis primarily due to high seasonal temperatures.
Total generation was down 5 per cent on the last quarter of the 2012 calender year, largely as a result of hydro generation falling 15 per cent as MRP built up lake levels at Taupo.
