Prime Minister John Key is scrambling to cover his back over a dodgy deal with Sky City, NZ First leader Winston Peters says.

Mr Key flew home from Singapore on Thursday night amid a storm of protest from opposition parties over the deal with Sky City to build a new convention centre in return for being allowed to have more pokie machines in its casino.

He says a tender process was held in the usual way, but Labour is accusing him of intervening in a way that shut out other bidders.

Party leader David Shearer says he has documents which prove Mr Key told government officials to stop working on a business case for a new centre after deciding to cut a secret deal with Sky City.

"It's no surprise that Sky City emerged victorious from the tender process because it was a one-horse race from the start," he said.

Mr Peters says there was nothing usual "about a shady backroom deal to change the law to allow Sky City to have up to 500 more pokie machines if it funds a convention centre".

Mr Key has rejected their allegations.

He says Sky City was the only one which could build a convention centre without government funding.