As that online form is not linked to on the actual complaints process page it probably goes nowhere now.
Complaints process with how to lodge a complaint:
I would say if you cannot contact Optus because you have a complaint then there web site is working perfectly, thats how it works, you pay good money, you get crap service, you can't complain.
Welcom to Optus in Australia.
Well technically speaking you can make a complaint to any of the telephone support staff if you wish. The official complaints process is via snail mail or the TIO if you think the issue can't be resolved. That online form in question has been a bit hit and miss over the years and it's obvious to me that Optus don't want anybody using it.