The Green Party says the Government's deal with SkyCity to fund and build a national convention centre could be unwound by a future government.
The Greens have sought legal advice over the agreement, which includes a compensation clause if any concessions are changed.
SkyCity will fund and build a $400 million convention centre in Auckland, and in return will be able to expand its gaming operations in the city.
Greens co-leader Russel Norman said on Tuesday that advice from Wellington-based law firm Franks and Ogilvie indicates that the Government is disregarding constitutional law with the deal.
He said the advice is also damning of the legal implications of the concession of extra gaming machines given to SkyCity, in that it sits uncomfortably with the rule of law.
Dr Norman said the opinion affirms the right of a future Green Government to revoke the SkyCity contract.