The Law Society says it appears the costs of the SkyCity convention centre deal outweigh the benefits to the public.
SkyCity is to build a $402 million centre in central Auckland in exchange for an extension of its casino license and being allowed to operate more gaming machines and tables.
The bill allowing the deal to go ahead passed its first reading last month and is now before a select committee.
Law Society representative Nick Crang told the committee it was difficult to verify the heralded advantages of the deal based on the publicly available information.
He said the legislation gives SkyCity a permanent, long term advantage over other casinos and allows further activity that Parliament accepts is socially harmful.
Mr Crang said the MPs must ensure they are absolutely confident the benefits of the deal outweigh the costs.