I changed from Telstra to Optus in January. I used to surf the web on my phone using 3G an easy 3 hours + a day without using 1GB of data. When I switched to the new phone I used a GB in 3 days. (I've since turned off all applications using 3G save a few and usually keep mobile data off). I'm not sure if this is necessarily the service provider -- it could be because I went from an iPhone 4 to an iPhone 6 (maybe photos are displayed at a higher resolution that uses more data or something - I have no idea. But I've definitely had a similar issue lol.
@ Everyone - same phone and prepaid plan for two years, unlimited texts, and don't call much. It's definitely a data issue and I swear my habits haven't changed much. My only other idea is that WiFi could be dropping out without me realising or Optus is playing silly buggers with data.