A barrister representing the Maori Council says the proposal by the Labour and Green parties for a single buyer of electricity could lower the value of Mighty River Power shares.
Earlier this year, the Maori Council was defeated in its legal attempts to stop the partial sale of the hydro-electricity generator.
Prime Minister John Key says the Mighty River Power share float is still on track, despite the temporary suspension of the share offer in response to the opposition parties' policy announcement.
The Government is also standing by its estimate that partial sale of three energy generators, including Mighty River Power, will bring in between $5 billion and $7 billion.
But the barrister for the Maori Council, Felix Geiringer, says the policy announcement looks likely to lower the share price if there is a change of Government and the Labour and Greens policy is fulfilled.