Sometimes Optus is generous anbd sometimes there's a lot of bluff and bluster.
At this stage I would slow things down (no pun intended) and WRITE a formal complaint. Stating that due to factors beyond both yours and Optus control the line speed is unacceptable. You would not have entered into the contract had you known that the deliverable speed was this low. State you want billing to be put on hold until the issue is resolved. You wish to end the contract now without penalty on either side. Wouldn't hurt to point out if you are still going to use the wireless broadband from Optus. Also that you will be making a complaint to the TIO if the issue is not properly resolved.
A written complaint has more legal weight at Optus. You'll get a complaint number and a formal reply (hopefully the contract is ended). If not then you can got to the TIO and they will work on your behalf. You may still end up being told to pay but chances are at some point along the line Optus will agree.