I can look on the Telstra website at my usage graph and I uploaded some data to an online backup as a test one day. I only had one device connected directly to the modem and the usage data on the graph was out by a few hours.
I see that the letter above gives only one time. So could you challenge that by providing a graph of activity that was up/downloaded on the graph that you can view from your ISP.
Example if I recieved a letter saying that I downloaded something at such and such a time and date, I could provide my graph from my ISP's website as evidence that it wasn't me. The activity I uploaded/downloaded in real time as the test I did earlier was between 10-11pm at night and the graphs activity on the Telstra's graph on the website showed the activity between the hours of approx 1-2am the next day.
If the time of the infringement is provided by the Detection service people is the date and time likely to match the exact time of the ISP and also the end users graph. As someone did mention all clocks etc would have to be synchronised.
Also If there was a dispute with the end users graph as displayed from the ISP's website and the ISP then the user could actually dispute internet usage charges from the ISP? So then one could potentially open a whole can of worms.