The National Party says tougher regulations on banks could cost families with a mortgage nearly $2000 extra per year.
The Reserve Bank yesterday confirmed that the big four Australian-owned banks - ANZ, ASB, BNZ, and Westpac - will have to raise the amount of capital they hold from 10.5 percent of their loans to 18 percent.
Smaller banks, including Kiwibank, SBS, TSB, and the Co-op Bank, will have to hold a minimum of 16 percent of capital.
The average minimum for the sector at present is about 14 percent.
The new rules will force the sector to raise as much as $20 billion in capital, to strengthen their financial position in case of any future crisis.
National's finance spokesperson Paul Goldsmith said that could force up mortgage interest rates from 0.2 percent to 0.8 percent.
He said even if they rose some where in between, at 0.5 percent, people would struggle.
"A lot of New Zealanders are feeling the pressure in terms of cost of living, we've had fuel taxes, we've had a number of things that are making it more expensive for Kiwis to go about their ordinary lives. So the only point we're making is these regulatory changes will add pressure on Kiwi families."
Mr Goldsmith said the Finance Minister should have insisted that a public cost-benefit analysis be carried out before the Reserve Bank made its final decision.
Finance Minister Grant Robertson yesterday said he did not ascribe to the view that the new rules would slow the economy, nor that it would be "a blow" to rural communities.