Alarming declines in patronage on Auckland ferries have been blamed on a double-counting blooper stretching back to 2011.

Passengers arriving at the Downtown ferry terminal on alternating indirect Fullers ferry trips from Waiheke via Devonport were counted twice, until the error was picked up late last year by a new employee of the ferry company.

But the newcomer rectified the error for monthly reporting purposes without telling anyone else, says chief operations officer Greg Edmonds.

The unannounced reversion to the correct script meant ferry patronage in January suffered by comparison with an inflated figure for the same month of last year.


It appeared to have fallen by 10 per cent, and by an even more dramatic 16 per cent in February, prompting Mr Edmonds to order an investigation.

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.