VAT Turnover Reported in GST Period

One of the my client has reported his VAT period Turnover during GST Period without payment of any GST liability. Further. amount of such VAT Turnover was also not reported in the concerned VAT Returns. Client is engazed in infrastructure construction business.
Because of so much confusion, client has not filed his GSTR-9 till date.
Please suggest whether GST liability will be payable or should be paid on the amount of VAT Turnover which is not reported in VAT Returns or GST Returns but posted in books during GST Period without booking and payment of any GST Liability on that amount ?
Please suggest with referring relevant sections of CGST Act.