Property Tax Levied by MCGM and recovery from member

Sir, Our Society was redeveloped and OC received in June 2017. We have two questions- 1) Builder got online NOC from Property Tax Dept and submitted to Building Proposal Dept and got OC. After builder gave possession to old members and few new members, Property Tax Dept of MCGM issued notice that their NOC was online and not valid and that builder has not paid property tax/ tax on land under construction of huge amount and that the Society has to pay the amount otherwise its property shall be attached. We wrote them letter that how did they issue OC when property tax was no paid and that if at all anything is payable its is duty of the builder and MCGM should recover it from builder directly. MCGM is refusing to take action against builder. What to do? 2) Property Tax Dept issued notice for property tax for period after OC which Society has paid. But builder is refusing to pay property tax and maintenance charges for unsold flats and flats. Please advise about the best and fastest course of action as Society has already paid property tax for the entire property. Thank you