Andy, banks and the NHBRC might not be qualified to make a call on who is doing the job properly and who isn't, but they make an excellent control point if they partner with an organisation (such as SAPCA) to police a list. Effectively the bank and/or NHBRC would be enforcing some admin stuff.

As to mandate, the bank would be protecting their interest (the building value forms part of their collateral, expensive maintenance costs resulting from shoddy workmanship in the original construction also affects the ability of their client to pay their instalments), and assuring quality in construction is part of the NHBRC mandate.