(It is not on website but you have to call them.)
Just was on call with Optus retention team and the lovely lady mentioned currently they have 50% off mobile plan for new numbers or port-in number for existing customers. I asked what if I move out all numbers and port-in and she mentioned yes you can technically unlock the discount.
So if you are existing customer and port-in back number or adding new number you unlock the discount of 50% on the mobile plan for the first 24 months.
I had called regarding currently in store specials 20% off on 12 month $49 60GB sim only plan and they offers $10 flat discount on each numbers.
Hope it helps someone.