I have almost exceeded my monthly 140Gig allowance. I still have 20 days to go. If I go ver the 140Gig, I get charged $10 per Meg.
Is there a way to avoid this? I.e. buy a prepaid sim for my computer? I currently have a large Optus dongle plugged in to my pc.
Assuming you're on the same plan that I'm thinking, you'll get charged $10 per GB (not MB) - so it's a reasonable charge, in my opinion.
If you'd like to avoid going over your limit, then removing your sim and replacing with a temporary alternative is your best option.
Have you checked to see whether vivid wireless is available where you live. Go to their website put your address in and see if the unlimited mobile broadband option is available cost is $90 per month and as they are part of Optus speeds should be sufficient for you.