The Performance, Delivery and Efficiency Unit (PEDU) said that the dispute between departments over who would take responsibility was “particularly disappointing”.
While officials faffed around, up to 1,600 homes were being damaged during the torrential downpours of June 27 this year when 44mm of rain fell during a three-hour period in the evening.
The Met Office calculated that the chance of this happening is just one to two per cent a year.
PEDU concluded that the human misery of flooded homes and the cost of emergency payments and repairs should lead to a review of investment in anti-flood measures.
Its report has recommended a shake-up of departmental functions, with the Rivers Agency moving from Sinn Fein’s Michelle O’Neill (left) in the Department of Agriculture to the Department for Regional Development headed by Ulster Unionist Danny Kennedy.
Mr Kennedy and Ms O’Neill publicly clashed when he said that she had “vanished” during the crisis — a claim she denied.
The report said: “... too much time was lost on June 27 on a debate around ‘whose water’ was causing the flooding.
“It is clear that there were differing opinions about which organisation should be in the lead.
“Eventually DRD agreed that it would be nominated as lead Government department. But this was done in exasperation at the debate rather than by way of acceptance that they were the correct lead department.”
PEDU also pointed out that Northern Ireland is the only part of the UK without a flood forecast and alert service, although the Met Office does work with other jurisdictions on flood alerts and rainfall forecasts.
“We understand that the Met Office did offer its extended service to Northern Ireland but this was declined as it was assessed by Rivers Agency,” it added.