Labour and the Greens are calling for the Government to disclose how many families will buy shares under its asset sales programme.
Green Party Co-leader Metiria Turei said many people would not be able to afford shares believed to be priced at $1000.
Mighty River will be the first of three state-owned power companies - with coal miner Solid Energy - to be partially sold to investors under the National Government's mixed-ownership model.
Labour's state-owned enterprises spokesman Clayton Cosgrove said the cost of shares would break the bank for most New Zealanders.
"The concentration of shares in the hands of a select few is a very real risk," he said.