They don't offer email support just the live chat on the link below.
Edit: I should also say they recently added a contact email at the bottom of the second link below however I don't believe this email address is meant for tech support so I would not expect a speedy response if at all. The live chat is your best bet.
Sorry to inform you, but Live Chat and E-mail mean nothing to Optus (ie. don't expect to get any results). I have been going through dramas with them for over 2 weeks now, mainly through Live Chat.
Here is a summary of what I have gone through recently with Optus....
SUN 27/1/19 - My HOME PHONE (LANDLINE) stops working.
MON 28/1/19 - I contact Optus (via LIVE CHAT on Optus website), and "Maria" books technicians for TUE 29/1/19, but no technicians come.
WED 30/1/19 - I open a case on the Optus website. Case Manager Christian Okeefe is assigned, and leaves a message "We're on it". This was 10 DAYS ago, and there has been NO REPLY SINCE.
WED 30/1/19 - I contact Optus again (via LIVE CHAT on Optus website). "Ada" books technicians for SAT 2/2/19 12:00-16:00, but again technicians DO NOT come (2nd TIME).
SAT 2/2/19 (16:00) - I contact Optus again (3rd TIME via LIVE CHAT on Optus website). "Clifford" assures me that technicians will come today (2/2/19) between 16:00-18:00, but no technicians come (3rd TIME).
SAT 2/2/19 (18:00) - I inform "Clifford", who now books technicians for TUE 5/2/19 - technicians DO NOT come (4th TIME).
WED 6/2/19 - I E-MAIL Optus at the e-mail address they provide on their website. Up till now, NO REPLY.
THU 7/2/19 11:10pm - I ring Optus. I am told that a technician will contact me within 20 minutes. Of course, this does not happen.
THU 7/2/19 11:40pm - I ring Optus again. "Laukik" tells me that technicians will come on TUE 12/2/19 - Guess what? Technicians DO NOT come (5th TIME).