SkyCity has since argued that construction cost inflation would reduce the size of the planned centre, and last year announced it would also spend $130 million building a 300-room five-star hotel adjoining the site.
Green Party co-leader Metiria Turei said the hotel decision meant SkyCity was able to capture additional benefits while also pleading poverty.
"On one hand SkyCity put their hand out begging for more money from the public, and on the other they are also millions and millions of dollars better off than the public knew about," she said.
Joyce said he was keenly aware of Turei's point, but these numbers were being accounted for.
"This is where Metiria and I are in danger of agreeing violently," he said. "The building of the hotel in that location does trigger a process by which we will be assessing its value."
He said SkyCity's revised building proposal would be compared with a similarly revised tally of benefits the company would receive after new plans were filed on April 30.
Joyce said this process would likely see SkyCity spend more on the NZICC than initially pledged.
"They've already indicated they were going to spend more on it than they were required. I'm almost convinced it [the final cost to SkyCity] will actually be more than the value of the concessions."
After initial suggestions that public funds might be used to help SkyCity build a more ambitious NZICC attracted substantial public criticism, the Government insisted the project be completed without resort to taxpayer funds.
The 2012 KordaMentha report, which helped sell the case to the public and Parliament, shows SkyCity may have downplayed its ability to take advantage of a potential boost in visitor numbers from convention-goers because of capacity issues.
"SkyCity says that its two existing hotels operate at very high occupancy levels and therefore there is limited scope to increase occupancy significantly," the report said.
The study said half of the more than 20,000 annual convention-goers would be expected to stay at SkyCity, but because of the capacity issues most would displace other visitors and only 10 per cent of this custom should count as a gain from the NZICC. The study said this much-reduced boost would add $5 million in annual revenue to SkyCity.
Weekend Herald calculations show that if SkyCity did not have capacity issues - such as with a new hotel - this figure would be $25 million.
A spokesman for SkyCity said there were no "definite" plans to build a hotel at the time the KordaMentha report was drafted, and the NZICC was "by no means the only factor in the [2014] decision to go ahead with the hotel".
The spokesman also firmly pushed back at demands from Turei that the NZICC deal be re-evaluated.
"There is no reason to revalue the contract with the Crown," the spokesman said, noting that the $130 million cost of the hotel was on top of the $402 million for the NZICC.
He said the NZICC still represented a substantial boon to the economy, saying other Auckland businesses would benefit by $90 million a year.
