I had a similar issue a while ago.

After some issues with Telstra's DNS I switched to using a third party DNS provider. It seems as if there is some issue with programs or applications updating with overseas DNS providers. Once I went back to Telstra DNS (they denied there was an issue but eventually admitted it had been fixed) data usage returned to its usual pattern.

Given Telstra seems to have made some system changes lately (I'm having trouble with email from my own domain going through their servers) there could be something funky they have done...