Meridian Energy says the Labour-Greens policy to introduce a single buyer of electricity will lead to a reduction in investment in renewable energy.
The company is 51 percent state-owned and appeared before the Commerce Select Committee on Thursday.
Meridian chief executive Mark Binns told MPs that the Labour-Greens proposal would make it more likely to invest in gas powered electricity generation because it would be cheaper.
Outside the committee, Mr Binns said more detail needs to be released so energy companies can plan for the future.
"We're lucky that we're in a low demand environment, so we aren't forced with making any investment decisions at the moment because we're in an oversupply position.
"But if we were in a position where we were looking at potential capital investment and, clearly, we'd need to see some certainty around this before we made any decisions."
Labour on Thursday rejected Meridian's claim over renewable energy. David Shearer, the party's energy spokesperson, said discussed the policy with Mr Binns on Wednesday.
"I feel that he is perhaps getting himself a little too involved in an election year in what I would see as politics ... 50 percent of his salary is being paid by taxpayers. Let's do this productively and appropriately, rather than how he's doing it right now, which is through the media."