Finance Minister Bill English insists Kiwibank will remain "100 per cent Government-owned" after New Zealand Post announced plans to sell off a share of the bank.
Mr English said keeping the bank in Government ownership was a "bottom line".
The Super Fund and ACC would not be able to sell any Kiwibank shares for five years. If they chose to sell them after this period of time, they would be offered to the Government first.
Mr English said the Government would exercise its right of first refusal.
The transfer of Kiwibank's shares to the two taxpayer-owned organisations would give the bank better access to capital, he said.
"ACC and the Super Fund have strong commercial incentives to see the bank succeed [and] grow its value because they are investment managers."
As part of the proposal, New Zealand Post would receive $495 million, none of which would go into Kiwibank, and the Government would receive a dividend. The size of the dividend was not yet known.
The changes would have little impact on Kiwibank customers, Mr English said.
The Labour Party also committed today to buying back any Kiwibank shares if in power.
Labour's SOE spokesman David Parker agreed that keeping the bank in public ownership should be a bottom line.
"National should also guarantee that if Kiwibank issues more shares to raise more capital to expand, the Government will take up its share to preserve NZ Post's 55 per cent ownership," he said.
He applauded Sir Michael's proposal, saying that National had refused to provide more capital for New Zealand Post or Kiwibank.
Green Party co-leader James Shaw said Mr English's commitment to buy back shares was "an empty promise" because he could not bind a future minister. The partial sale was a "slippery slope to privatisation", he said.
Mr Shaw said the deal did not improve access to capital because all of the proceeds would go to New Zealand Post. His party wants Kiwibank to get a $100 million boost in capital to make it more competitive with Australian banks.
Sir Michael criticised this policy today, saying that the Government would be better off putting money into building houses.