If GST Returns are not filed within time, you will be liable to pay interest and a late fee.
Interest is 18% per annum. It has to be calculated by the tax payer on the amount of outstanding tax to be paid. Time period will be from the next day of filing (26th/ 29th Aug) to the date of payment.
Late fee is Rs. 100 per day per Act. So it is 100 under CGST & 100 under SGST. Total will be Rs. 200/day. Maximum is Rs. 5,000. There is no late fee on IGST.
**Note: Late fees can be changed at anytime by GST directives. Please read details on gst.gov.in.**