What is eating up all my bandwidth?

After upgrading my internet connection from 512kb/s to 1.5Mb/s (and 14GB on-peek/month to 20GB on-peek/month), our download usage has gone through the roof. I have made special effort to be careful with downloads and to not download much, but our usage will just not go down. Before we would download approx 600 Mb per day, and it has skyrocketed to over 1 GB per day.

I contacted my ISP about this, and they said all they can do is to get one of their network technicians to look through our logs, and that would cost $88AUD per hour (which would be refunded if the fault was on their end), which I am not willing to pay.

What other options do I have? Being in Australia, are there any legal routes I can go down? Someone else cannot be using our Wi-Fi because not only is it secured, we live in the middle of nowhere and we would see someone if they were leeching off our connection.

I thought about updaters and game consoles running in the background, but with a new Macbook being the only addition to the hardware in our house (which replaced another computer), there would be no reason for our usage to skyrocket.

As previously suggested, installing monitoring software would greatly help track the source of this problem. Although I have a feeling a lot of your problem is Youtube and other streaming video sites, my siblings are horrible for this as well. It doesn't seem like a lot, but 1 hour of viewing can easily rack up 150mb+ bandwidth on regular videos, not counting if someone is viewing HD videos. HD videos are much worse, more than likely 2 to 3 times the bandwidth per video.

Having an old speed of 512k would probably deter other residents from Youtubing because of constant pauses in the video makes it annoying to watch. The 1.5Mb/s jump would fix that, and I would assume a lot more streaming video is being watched.

If the Macbook was used, you may want to look into the applications installed, make sure no malicious software is running in the background gobbling up bandwidth as well.

How about running something like FreeMeter on each PC? Is it possible bit torrent is running one one of your PCs? Leaving websites open in your web browser can use up some bandwidth, but not the amount you describe. I'd try seeing what your individual PC usage is and how their totals compare to those being give by your ISP.

As an extra precaution I'd do some serious spyware/virus/etc scanning. It's possible one or more of your PCs can be infected by something that is using a lot of bandwidth without your knowledge.

There are many websites that reload themselves, or parts of themselves at set intervals, and this can eat up bandwidth.
noscript plugin (firefox) helps me keep websites from using my cpu and bandwidth senselessly