"We were not seeking to be the definitive culture consultant. It's ultimately for boards to do that work for themselves," APRA chief Wayne Byres told the banking royal commission on Friday.His three hours of evidence in the morning was the last that will be heard in public. On themes around culture and prudential supervision, APRA is almost as punch drunk as the banks it oversees.Michael Hodge, counsel assisting, put it to Byres: "in an assessment following the prudential inquiry [into Commonwealth Bank], APRA had considered that in its ordinary supervisory work before the inquiry, it had been less effective at driving cultural change at CBA?""Yes. We were battling hard, yes," Byres said."We had had some successes, but we were continuing to battle hard there."Summing up how the banking regulator is now approaching the fashionable (and in the past insignificant) theme of culture, Byres said "we can have multiple levels of intervention. The day-to-day supervisors will be forming views but they're not necessarily experts on culture and it will be sort of intuition and gut feel and experience."Prudential reviews with the firepower and exposure of that imposed on CBA last year is the nuclear option.Some in-house expertise is gaining depth, most of all "to bring an important element of cross-industry perspective," Byres said."If we felt we had a real problem, or this work that we had done had come to the conclusion that there was something very poor about the risk culture of the entity, from going and doing one of those deeper exercises."Byres made clear to Hayne that "we have sufficient powers that if we thought we wanted to go and do one of those really deep sort of exercises like in the CBA review, we could do that."It would just require us to hire some external resources to help us do that exercise."Some drastic measures may yet be needed. "Do you think that the boards of APRA-regulated institutions have now become quite comfortable with the requirement that they form a view of the risk culture of their institution?" Hodge asked."No," an answer Byres then widened."If you looked at any of the major banks, there actually isn't some monolithic culture. There is some sense of organisational culture."The challenge I think they all feel is they recognise the task, they accept the task."