registration

We have a composite order for design, supply, erection and commissioning of a water treatment plant at Ashok Leyland H2- Hosur. We are currently registered for GST in Pune and Delhi. All material would be supplied in CKD/SKD condition from Pune and billed with IGST against the HSN code for supply. For the civil works and erection, we would be hiring local contractors in Tamil Nadu and would like to bill the same with IGST against SAC code. We request your advice on going ahead with this contract without the need of getting into a casual GST registration in Tamil Nadu as discussed. Request kindly advise.