If the change had continued to go undetected, a drop of 11 per cent would have been reported for March.
But an adjustment of the comparison with last year showed a small increase in ferry passenger trips for March, up 0.4 per cent to 494,123.
"A new employee had found a spreadsheet error and corrected it, but didn't advise us," Mr Edmonds told a meeting of Auckland Transport's board, promising directors that his staff would reach back to 2011 to amend passenger numbers in light of the mistake.
Auckland Council infrastructure chairman Mike Lee, a transport board member, questioned a press release issued in January blaming bad weather for the apparent downturn.
"It wasn't bad, it was better than last January - believe me, I went out on the water," he said.
Although the Waiheke and Devonport ferry services are unsubsidised by Auckland Transport, Mr Lee said he was concerned about a decline in wharf taxes collected as levies on tickets.
"If Waiheke is enjoying moderate [patronage] increases, what's happening to the wharf tax?"
Mr Edmonds said that Auckland Transport had drawn on MetService information in preparing the press release, but would continue to investigate taxes collected by various Waiheke ferry operators.