My issue appears to have been resolved in my favour. Salesman had made notes on computer of our conversation so although discount shouldn't have been promised, it clearly was, and so will be honoured. A case rep is monitoring my account to ensure that the discount gets applied (by another department).
I had found the website info confusing so I'd kept notes myself with names and dates, I summarised or confirmed info to be certain I understood. I was prepared to contact the TIO if necessary with this documentation, said so in a letter written to Optus Customer Relations department. Phone calls and online chat had proven useless.
Customer Relations staff member was excellent, calmed my ruffled feathers, followed up, and kept me informed. Pity it had to escalate to that level.