Intetest on belated file of GSTR3B

Sir i have filed my GSTR 3B for the tax period April to December 2018 in January 2019. Is i am liable to pay intetest on the unpaid amount remain due from it's due date. Is late fee is aplicable in belated return.? Please reply..
Tax Consultant

for April 2018 to September 2018_No late fee for belated return filing in January 2019. Late fee applicable from October 2018 returns onwards.
Yes, Liable to pay Interest @ 18 % on GST due and payable till date of payment of GST