Data used without my knowledge. Excessive Bill.

I have a data package used in my wifi device. I do not use it excessively. On the 17th March, I did not use my device as I was at work, and was out that evening until 11PM, I DID NOT connect to the wifi at all that day. I received an SMS notifying me that I have reached my voluntary call limit. on 19th March I received a sms from Vodacom about excessive data usage R5870.93 (over 6GIGS) I did not use this data!!!!<br> <br> I lodged a enquiry SC-80J2C-C9Z99