I'm having problems with my new Telstra cable as well.
My computer and laptop were turned off at about 11pm I came home that night at 4am turned on the computer and checked the usage on the bigpond side 250megs.. from where?
I been noticing my usage flying up! I think optus is better.

 

Does telstra provide a "by the hour" usage graph? (iinet does)
If so, did you check it and are sure the dl occurred whilst the computer was off?
Do you have a wireless router/modem?
If so, did you setup security for it? someone may be stealing internet from you.

 

Windows updates by any chance? Or other software automatically downloading updates behind the scenes?

Or a virus on your machine?

 

Yeah I say its an infected drone. You are probably spamming millions of people every time you log on.

 

Nah it updates everyday, so by midnight it should be 0 for the new day, it has a daily graph, anyway midnight to 5am is 5 hours my computer is off! This happened again 2 nights ago, I turned everything off before I went out which was at like 9 earlier! I was downloading heaps, before that. Anyway I took out the power cable turned off computer all that, came home at 5am checked, 1gb?? I think they are dodgy, if someone was using my internet the modem would have to be on right?
Not every time this happens, as I do go out late night alot I do catch the internet downloading nothing, sometimes I do same thing I come back and its like 90mb, which is still alot for an off computer, and sometimes its 1gb and thats when it pisses me off. I havnt even downloaded much this month yet I am on like 90% usage and I still have two weeks left. thats with 25gb a month

 

we have gone to the virgin wireless broadband, and it still seems to be happening .

going to have to take the laptop to my mate to have a look at it this weekend .