Government officials were told to stop working on a business case for a new convention centre in Auckland after Prime Minister John Key cut a secret deal with Sky City, Labour says.
Party leader David Shearer says he's got cabinet documents that prove Mr Key directly intervened in the tender process.
"This looks like a secret deal was done behind taxpayers' backs," he said on Thursday.
"It's no surprise that Sky City emerged victorious from the tender process because it was a one-horse race from the start."
Mr Shearer says officials wanted to develop the business case but when Mr Key heard about the Sky City option he ordered them to stop working on it.
Labour, the Greens and NZ First say the tender process must be reopened because there was political interference.
They say Mr Key personally struck a deal with Sky City for it to build the $350 million convention centre in exchange for being allowed to have more pokies in its casino.
Mr Key is rejecting all their allegations.
"It went through the normal tendering process, Sky City was the only bidder prepared to look at a deal that didn't involve government resources," he said just before boarding a plane in Singapore to return home.
"They can run around as much as they like looking for conspiracies but they're never going to find one."
Mr Key says he approached Sky City, and four other potential bidders, on his own initiative.
"No one suggested it to me, I did it," he said.
The deal with Sky City is close to being finalised and the government says details are commercially sensitive.
Labour says Sky City wants 500 extra pokies and the government is offering 350.