Vodafone Postpaid Internet bill too much too little data usage.

I am Akshay Pawar from Nashik and my dad
Vinaykumar Pawar has a Vodafone postpaid sim card, so for the May 2018 month he was billed for Rs. 507 As this bill was unusual for us I checked the bill there I saw that my dad was billed with Rs. 325 plus Rs.78 as tax for using just 79 MB of Internet data. I know we don’t have a plan that allows us to use some free Internet usage. But Rs,. 404 for using just 79 mb of data is waayyyy toooo much. Hiw can they bill us for such a little data.