Although not unheard of phantom usage appearing on a customers usage meter is very uncommon. In most cases the source of an increase in usage can be tracked back software or a device in use.
One of our superusers put together a list of programs that you could use to track usage independently here https://crowdsupport.telstra.com.au/t5/Home-Broadband-KB/How-to-Independently-Measure-your-Bigpond-Usage/ta-p/73694 that may help you identify the cause. I cannot recommend a specific program nor does Telstra support these programs so you will be using at your own risk.