Western Australia and Queensland have the highest proportion of homeowners with mortgage debt that is equal to or larger than the value of their property, a survey has found.

Roy Morgan Research on Thursday said 311,000 homeowners, or 6.8 per cent of all owner-occupiers with mortgages, had no equity in their properties.

That means these households believed their property was worth the same or less than the amount they still owed on their home loan.

It came as National Australia Bank executives told a parliamentary inquiry the bank was taking a more cautious approach to lending for inner-city apartments in Melbourne and Sydney, where several experts have also warned of the potential for an oversupply of units.