Everyday borrowers could struggle to get bank loans unless the corporate regulator urgently issues fresh guidance on how banks satisfy tougher responsible lending duties, after the royal commission interim report found verification of borrower expenses must be improved but stopped short of saying what is required.

The Australian Securities and Investment Commission has guided lenders on how to comply with consumer credit laws through its Regulatory Guide 209, but this document has not been updated for four years and is cast in very vague terms. Bankers say this is one reason for defaulting to indexes or relying on customers' estimates of spending – which Commissioner Kenneth Hayne said is no longer rigorous enough.