The chief executive of the Ulster Bank told a joint meeting of assembly committees that 300,000 of its 800,000 customers had been compensated for the bank's IT failure, on 11 October 2012.
Thousands of customers were unable to withdraw cash or access their accounts for weeks after a software upgrade in June 2012.
Jim Brown told a joint meeting of the Finance and Enterprise committees that customers had received a total of £18m in compensation so far.
"We are deeply sorry and apologise unreservedly to all involved," he said.
Mr Brown said the IT failure had been "a major disaster" from the bank's point of view.
Daithi McKay of Sinn Fein said members had the impression from Mr Brown, after his committee appearance on 5 July 2012, that an announcement on compensation would come within days.
Mr McKay said eight weeks passed before an announcement was made on compensation, "and I think that's absolutely scandalous".
"We felt extremely misled by the comments in July and August," he added.
Mr Brown said "the timeframe" took longer than the bank had expected.
The DUP's Robin Newton said the Federation of Small Businesses had described the general perception of the bank's compensation offer as "derisory".
Bank official Stephen Cruise said the initial emphasis following the compensation announcement had been on a figure of £20.
Mr Cruise said the average payment was £60.
Paul Girvan of the DUP asked about branch closures.
Mr Brown said the branch network was always always under review and many customers were now using electronic banking.