The finance minister said he told the chairman of Royal Bank of Scotland to "be honest with people" when it came to ongoing problems with Ulster Bank, on 3 July 2012.
Sammy Wilson was responding to an urgent assembly question tabled by Sinn Fein's Phil Flanagan.
It related to the problems customers faced accessing their accounts, including wages and benefits, following a technical fault.
The fault was caused by a software update, and initially affected millions of customers within the Royal Bank of Scotland (RBS) group, which includes Ulster Bank and Nat West.
Mr Wilson said he met with Sir Philip Hampton and "relayed disappointment" that it was still not clear when the problems would be fixed.
"The bank has probably done itself some damage as well as raising problems for individuals by this drip-feed (of information)," he said.
"It may have been better off if it had been upfront and said it was a three-week problem."
The minister said he also told the RBS chairman he had been disappointed by the initial lack of response in finding ways of dealing with customers' requirements for money.
Mr Flanagan said there was obvious "anger and frustration" among customers.