SkyCity will have made $27 million out of the convention centre deal by 2017 without even turning the first sod.

That's the claim from the Green Party based on figures from the Parliamentary Library, which calculate the impact of inflation on gambling concessions fixed at 2013 rates.

The building costs, which were not fixed, are hit by inflation, meaning the taxpayer gets less the longer construction is delayed.

"Our research shows the value of the concessions is going up while the value of the convention centre is going down," said Green Party co-leader Metiria Turei.

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The $402 million cost to SkyCity was balanced against gambling concessions, giving the casino company an extension on its licence monopoly through to 2048.

In return for building the convention centre, the casino was able to get a law change letting it secure and expand its operations into the future.

The figures done for the Green Party used the June 2013 to June 2014 rate of inflation - 1.6 per cent.

A year after the deal was signed in June 2013, the concessions were worth $408 million to the casino. By 2017, the value would be $429 million.

In contrast, SkyCity has blamed increases in building costs and a grander design for a $130 million cost blowout - a "cost gap" the Government knew of just months after signing the deal but never revealed until the casino company told investors in December.

Ms Turei said there were too many parts of the deal that had shifted to the advantage of SkyCity.

"We're calling for the deal to be revalued - at a minimum."

Last year, the Herald revealed SkyCity was using land it had bought from TVNZ for the convention centre to build a new hotel to soak up increased demand.

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Papers released through the Official Information Act showed officials believed the land was "more valuable" and the "value proposition needs to be adjusted".

Officials drew up "suggested draft terms of reference" for a convention centre "evaluation" but never went ahead with the study. Other changes have also emerged, including the capacity of the Plenary Hall.

The ICC agreement specified a 3500-seat dedicated Plenary Hall, whereas the resource consent documentation shows a design with 3000-seat capacity.

Economic Development Minister Steven Joyce is adamant nothing will be done to revalue the deal.

"No formal revision of the economic benefits has been undertaken."

He said the information used to assess value had not changed "significantly".

"The Crown will retain the right to reject any amended designs submitted by SkyCity if it does not think an international-standard convention centre will be built or that the Crown does not achieve sufficient value for the concessions."

Mr Joyce pointed to comments by SkyCity chief executive Nigel Morrison saying it would "likely" spend more than the $402 million, and said he awaited its "revised proposal".

He said gatherings larger than 3000 could meet in the main exhibition hall.