The five largest power companies did not manipulate the electricity market, which hit a seven-year high in prices late last year, the Electricity Authority says.
A group of small retailers selling at the wholesale price, including Electric Kiwi, Flick, Pulse Energy, Vocus, and lines company Vector, laid a complaint with the market watchdog that the big generators had taken advantage of conditions to maximise prices which was an "undesirable trading situation".
But Authority chief executive James Stevenson-Wallace said no such situation occurred.
"We did not find any evidence of anti-competitive behaviour. We have absolute confidence in the wholesale market.
"There is no disagreement that wholesale market conditions were unusually high in September and October 2018," he said.
Prices hit as high as $1000 a megawatt per hour late last year, averaging three times the normal wholesale level because hydro lakes were low, while backup gas supplies were restricted, and demand had risen.
The top generators, Contact Energy, Meridian Energy, Genesis Energy, Mercury, and Trustpower, denied the allegations and countered that the small companies had been inadequately prepared for the price rises.
The small retailers lost customers and some stopped selling because of the price spike.
"Any participant in the market needs to understand those risks, plan ahead and manage them on behalf of their customers," Mr Stevenson-Wallace said.
However, the Authority said there were issues with information disclosure.
"This investigation has reiterated the need for better disclosure of fuel information and improved tools to support liquidity in the market."