interest on delayed pmt undet GST

S M Gala (Head: F & A) (100 Points)

23 June 2018  
dealer who failed to pay n file it's 3B from SEP 17 to Mar 18 has filed all this pending returns with late fee per 50 rs per day per return.
Now, shall be liable for interest on delayed pmt? if yes how to pay n discharge this liability?