The scheme means investors will receive one additional share for each 25 they bought in the initial offer and held for 24 months. The bonus shares are capped at 200 per investor.
Mighty River shares were trading at $2.37 this afternoon - 13c below their issue price. At $2.37 each, the 377 million shares bought in the retail offer are now worth $50 million less than investors paid for them.
Treasury's admission comes just a few days after it revealed to the Green Party that half the shares in the retail offer went to just 13,000 investors and 10 per cent went to just 400 who made an average investment of almost $250,000 each.
Today's acknowledgement that companies trusts and investment institutions were able to buy shares in the retail offer "is a further nail in the coffin of National's myth that it was selling shares to ordinary Kiwi 'mums and dads'," Green Party Co-leader Russel Norman said.
"That was always a con, and the facts prove it."
Dr Norman questioned whether Prime Minister John Key really knew at all how many ordinary New Zealanders bought Mighty River shares.
"Any further asset sales will only be worse in this regard because Mighty River Power has already given New Zealanders a chance to buy into a partially-privatised electricity company and they overwhelmingly rejected it. If ordinary Kiwis didn't want to buy into Mighty River Power, who's going to buy Meridian?"