The Labour and Green Parties will outline their plans for changes in the electricity sector later in the week.
The Labour leader, David Shearer, has warned potential Mighty River Power investors his party would reform the electricity sector if it leads the next Government.
The Prime Minister, John Key, said Labour had a responsibility to tell New Zealanders if it planned to make radical changes.
Mr Shearer said he would hold a joint news conference with the Green Party on Thursday, where both parties would outline their policies.
He said Labour's policy would be fully costed, and aimed at bringing power prices down.