GST on bank audit

If bank audit is done in Karnataka state, and while billing GST if dey give gstn of Chennai state,then whether sgst,cgst will apply or igst will apply?
OR
Like, if v provide bank audit service for the bank branch located in Karnataka and the banks headoffice is in some other state (say Chennai) ,then while Billing GST, whether SGST and CGST we should apply or IGST billing?