GST not collected for prior contracts by contractor from PWD (Govt.) Dept.

Sir, I am M.P. PWD contractor, making roads, dam, bridge for government department since 28 year in Khandwa district madhya pradesh.

Prior to GST (01.07.2017) for Govt. Contractor there is no Service tax and VAT is also paid advance which is refunded after entry tax adjustments.

All the Bills of Government contractor are finalized & made by PWD dept. (only estimates are provided by contractor).

After GST, PWD dept. is not mention anything (GST/VAT) in bills of old contracts. Therefore, there is no tax charged by contractor towards the supply amount.

On new contracts entered after GST (01.07.2017) Total amount paid by Govt. Dept. (Receiptant of services) is Contract amount + 12% GST.

Now, I have followeing queries towards GST Implimentation -
‎
1. What is present tax structure for work contract which are contracted prior to GST (01.07.2017) ?

2. What are the provisions for Government contractors (PWD Contractors) under GST ?

3. ‎If GST is applicable on prior contract, then what should I do.. when Govt. Dept not paid me any GST amount on such contracts ? Should I issue debit notes on Govt. Dept (receiptant) on amount of GST ?