Wow what does that $140 relate too? I certainly can't see it being a late fee.
You can certainly dispute things but as mentioned, a late fee wouldn't exceed $20.
Check your bill (paper, email or here on line) and see what it says you have been charged for.
Possibly excess data ($10 per Gb) or a piece of equipment (modem?) for $100?
Phone or Livechat (24/7 on this forum). First thing is to figure out what the charge is for.
If Optus can't provide a suitable answer then an official complaint to OPtus is needed (detailing the issue and resolution desired.) - you'll need to write a letter.
If after 10 days you don't have a suitable repsonse then on to the TIO.