"In the bad old days, you'd pay horrendous interest rates on second mortgages," she said. "That'll happen again."
Ms McMorran predicted dodgy finance companies like those which had collapsed in the global financial crisis could return to the market.
Prime Minister John Key this week told the Local Government New Zealand conference in Hamilton that first-home buyers remained "a priority" for the Government but "all tools have to be considered" in balancing their needs with the Reserve Bank's likely restrictions on home lending for buyers with small deposits.
He backed the Reserve Bank's concern about the emerging housing bubble.
And he stopped short of endorsing earlier suggestions by Housing Minister Nick Smith that one way to assist first-home buyers would be to give them greater access to funds in their KiwiSaver accounts.
Reserve Bank discussion documents show it is hoping the higher cost of non-bank loans will turn people away from "opportunistic lending".
Reserve Bank Deputy Governor Grant Spencer said recently that the most obvious way for first-home buyers to top up their deposit would be to hit up mum and dad or other relatives.
Bankers' Association chief executive Kirk Hope said it was likely buyers would turn to sources other than banks for the increased deposits. "In Canada and Sweden that's exactly what happened," he said.