Shearer said governments should be able to make changes to legislation but he would not make a commitment to stopping the deal, saying he would look at the fine print.
"To guarantee 35 years out that this legislation is to be set in stone for that length of time is wrong."
A KordaMentha report on the deal shows that in the event regulatory concessions are changed by the Crown before the deal ends in 2048, compensation will be paid to SkyCity.
Its assessment of the value of license renewal is between $65 million and $115 million.
Greens co-leader Metiria Turei said the deal was "disastrous" but she had not yet sought legal advice on the implications of rejecting the deal.
She said it could become a bottom line in future coalition discussions between the Greens and Labour and the Greens would want the legislation repealed.
"Whether we can negotiation that with them in the future - we will see. The Green Party does not accept being held to ransom like this and will repeal this legislation if we get into Government," she said.
"One of the worst aspects of this deal is the attempt by John Key and the National Government to hold future governments to ransom - to lock future Governments out of making public policy regulation to benefit or protect New Zealanders who are suffering from problem gambling caused by the casino.
"We consider this to be unconstitutional and it is a breach of Parliamentary sovereignty to lock a government into a commercial deal for 35 years."
