I'm using Onavo and 3G Watchdog at the moment. Both report results within 1-2 MB of each other. I prefer Onavo because it gives app break downs and will also give you alerts if background apps start using a lot of data. It can also notify you when you install apps if it is a known high data usage application.
I use RadioOpt
I think it's called Traffic Monitor, with the developer being RadioOpt :P
I have seen many people panic over this exact same issue with Telsta. In fact when I went onto my new contract I got an SMS from them saying I was over 90% of my usage and I had only used it for 2 days. I was thinking, how the hell did I just use 900MB?
Telstra often bring people into the middle of a billing cycle, and they set your data usage as a percentage of that cycle that has already elapsed. So there is a chance that you never actually used that amount.