Green Party co-leader Metiria Turei says John Banks should abstain from voting on the SkyCity convention centre deal.
The $400 million deal would give the casino more gaming machines in return for building the convention complex.
Mr Banks has resigned his ministerial positions as he awaits trial on a charge of filing a false electoral return.
SkyCity was one of the donors to his 2010 campaign for the Auckland Mayoralty who he claimed was anonymous.
Mrs Turei says other MPs have abstained from voting where they believe there's a conflict of interest. She told Morning Report it's a matter of Mr Banks' personal integrity.
Her comments come amidst increasing pressure on the Government over its support for Mr Banks from opposition parties.
Parliament held a snap debate on Thursday over Mr Banks's resignation as a minister over the charge he filed a false electoral return after the 2010 Auckland mayoral election.
Opposition parties said the Government's integrity has been damaged by its continued support for Mr Banks.
But Health Minister Tony Ryall reminded Labour it misused taxpayers' money to pay for its election card pledge in 2005. Mr Ryall said it could not lecture the Government on ethics.