New Zealand First might want the government to buy back BNZ and meld it with Kiwibank - but there's limited evidence that it would work.

"It would require a number of Kiwis to front up with a whole bunch of cash to buy a new bank," an economist says.

Michael Reddell said he "blanched" when he heard Winston Peters' promise to buy back BNZ.

New Zealand First might want the government to buy back BNZ and meld it with Kiwibank - but there's limited evidence that it would work.

Critics argue expanding Kiwibank with private capital would be a better option.