Interest on Late payment of GST

Yes Sir the Department's Interest Notice is correct...

Even if you manage to pay the taxes within the due date of filing Return / depositing taxes, but file your GSTR-3B post deadline of filing the Return, then both Interest @ 18% p.a. along with Late Fees would be applicable. So you need to pay the Interest...

Thanks for responding, I've a doubt here. what is the due date of payment for tax in general?

The due date of payment of tax and filing the GST Return in general is 20th of the following month ( 22nd / 24nd incase of Small Taxpayers having place of business in the Category I or Category II States ). The tax can be paid / deposited earlier but the GST Return should be "filed" within the due date.

Note that the GST Return shall be treated as a "VALID RETURN" only when the due tax has been deposited successfully and the Return has been filed.