The biggest ship to visit New Zealand waters will cruise into a spending squall when it docks in Auckland tomorrow.

Protest group Stop Stealing our Harbour has seized upon differences between Statistics New Zealand and the New Zealand Cruise Association on how much ship visitors spend when coming to the country's biggest city.

The group is fighting Auckland Council plans to install "mooring dolphins" off Queens Wharf, which would allow big cruise ships like Ovation of the Seas to berth rather than anchor in the harbour with passengers and crew forced to take tenders to shore.

The 347m long Ovation of the Seas, which visited New Zealand last summer, is back in Kiwi waters and is due to arrive in Auckland tomorrow. The $10 million mooring dolphin structures would be 80m-85m, linked to the end of the wharf by a gangway, and are scheduled to be in place by the 2019/20 cruise season.

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Stop Stealing Our Harbour's Michael Goldwater earlier this month questioned the methodology behind estimates of the economic contribution that cruise ships bring to the city.

Auckland Tourism, Events and Economic Development (Ateed) said that in the absence of firm data it had previously used calculations from reports commissioned by NZ Cruise.

An August report from Market Economics, on behalf of NZ Cruise, estimated that visiting ships spent $419m in Auckland during the 2017 season.

Statistics New Zealand, on the other hand, estimated in accounts released last week that cruise ships spent about $96m in the city in the year to June 2017.

Goldwater said the gap between the reports was "wider than the Grand Canyon" and he was alarmed that council had relied on the earlier estimates.

Ateed's Steve Armitage said that Stats NZ information did not capture what crews spend, or the airfares and the spend of passengers staying in accommodation when they were either starting or finishing their cruise in Auckland.

Stats NZ also did not capture information about domestic visitors or those on a New Zealand passport, or the value of cruises which did not leave New Zealand waters.

"Statistics New Zealand have done a good job accessing hard data which previously had to be based on modelling, however, there needs to be further in-depth analysis of where the differences lie and why," Armitage said.

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NZ Cruise chief executive Kevin O'Sullivan said there was different methodology around card payments for Stats NZ's account.

"There's a very wide range of figures that are included in Market Economics data which is not included in the information that comes out from Statistics New Zealand.

"There's nothing wrong with it, it's just a different approach," he said.

O'Sullivan said Auckland had already had to turn away a large ship because it did not have the facilities to allow it to berth.

"The benefits we see, not just downtown, but throughout the Auckland region from these large ships just won't occur if the dolphin is not built."