Soaring home prices in Australia's biggest cities don't necessarily mean the country is in the grip of a housing bubble, according to the heads of the big four banks.

Testifying at the parliamentary inquiry into banking this week, the chief executives of National Australia Bank, Westpac and Commonwealth Bank all said that while they are worried about elements of the housing market, prices aren't over-inflated.

"I would draw the distinction between a speculative bubble in prices and prices beyond what fundamentals would justify," Westpac's Brian Hartzer told the parliamentary committee on Wednesday. A bubble isn't occurring in Sydney or Melbourne, where house prices have risen the most, he said.

"There are increasing risks, but I still believe the answer is no," National Australia Bank's Andrew Thorburn said when asked if houses in Sydney and Melbourne are overpriced.