"We had a referendum - it's called a general election, and National won that election on the back of this major policy plank with an overwhelming majority, the biggest result we've received in MMP history," he said. "It's a complete and utter waste of money because it's just about sending a message."
Asked if he would ignore the referendum result, Mr Key said it might be too late because the mixed-ownership programme was already going ahead.
He would not postpone the Government's float of Meridian Energy until the referendum was held.
"I think it will be fairly politically driven anyway - the turnout is likely to be relatively low, and those that vote are more likely to be those who are opposed."
Green Party co-leader Russel Norman said the referendum should take place as soon as possible, and the partial sale of power companies should be suspended until the vote was held.
He defended the cost of the referendum, which Mr Key said could be between $9 million and $30 million.
"Let's put it in perspective - the Government has spent over $100 million already on an asset sale programme which is extremely unpopular," he said.
Labour's state-owned enterprises spokesman Clayton Cosgrove said Mr Key should respect the democratic process, and the asset sales programme - including the Meidian float - should be halted until after the referendum.