Meridian Energy has been rapped over the knuckles for ramping up prices during a power shortage last year.
The Electricity Authority (EA) said the company used its market position to offer electricity about $5000 a megawatt hour when prices more normally would be about $50.
The authority's chief executive, Carl Hansen, said Meridian's conduct was not acceptable but it was the first time the rules have been tested.
"We thought it appropriate to give them a warning letter," he said.
"And we are taking some actions in terms of reviewing that part of the code to see if we can provide some further guidance for the parties that are operating in the market.
Mr Hansen expected generating power companies to take note of the decision and make sure there would be no repeat of such behaviour.
A Meridian spokesperson said the company welcomed and accepted the authority's decision.
"The factors involved are complex and the decision-making process is important, so we will be seeking clarification from the EA to better understand this decision and its implications over the coming days and weeks," they said.