The nation’s largest home lenders, Commonwealth Bank and Westpac, have split from their peers and the industry lobby group, backing conservative borrowing rules as others complain they are locking first home buyers out of the market.

The two biggest mortgage banks told a parliamentary hearing on Thursday that any changes to the so-called serviceability buffer – a rule requiring them to assess if borrowers can repay a loan at an interest rate 3 percentage points above current levels – would be inappropriate.