I cancelled my broadband account back in Nov/Dec 2018 and was told yes all cancelled etc yet I'm still being billed for this account months on. I asked was there anything else I needed to do and was confidently told no that's it now you're up to date on payments etc.
To my dismay the direct debit came out again in Jan 2019. I cancelled the direct debit from my bank but am still being billed via email for a broadband account I haven't used since Dec.
When you say you have "cancelled the direct debit from my bank" do you mean you cancelled it at your bank? While banks will generally ask you to contact Optus you can cancel any direct debit from the bank end.
Otherwise I would suggst you write a complaint to Optus and ask what's going on. Its a much more legal process so you should get either an apology or an explanation in 14 days.
Still not really clear. Has Optus taken any money or just sent you a bill? Sounds like the later. Definitely something to contact Optus directly and file up.