The Greens believe they've exposed another taxpayer handout to SkyCity over the Convention Centre deal.

Documents they've obtained under the Official Information Act show ministers were told last year the casino operator's plan to use former TVNZ land as a hotel, rather than a convention centre, made the land more valuable.

Party Co-leader Metiria Turei says officials advised the deal be re-calculated as a result but there's no evidence this happened.

She says its more evidence how much a dog the deal has been, but Prime Minister John Key disagrees.

"If a convention centre can be built in Auckland, along with another big hotel there, that would add to what would be an impressive part of downtown Auckland and service the economy well," he said.

Labour's Andrew Little told him there are contrasting view between Mr English and the deal's chief negotiator Stephen Joyce.

"Whose advice will he take on this issue," he asked. "Bill English, who is trying to save the budget, or Steven Joyce who is trying to save his backside."

"I don't like to choose between the children so I'll take both of their advice," Key responded.

Julian Mace of the Institute of Quantity Surveyors says SkyCity's claims of building cost escalation don't stack up.

"Christchurch and particularly Auckland are experiencing some escalation but certainly wouldn't account for anything like that sort of value," he said.