The whole world knows that I bought a new phone. (Well, if you aren't already aware of it, that is because you weren't paying attention. That information is public knowledge.)
The plan that I signed up for was the Optus $85 My Plan Plus with a $20 discount per month for the entirety of the 24-month contract period.
How postpaid works in Australia is different to what the word mean. You have to pay the minimum amount (in my case, that should be $65) in arrears, and if you go above that minimum amount (eg: you used more than 7GB data or made international calls exceeding the 300-minute allowance etc.), the extra amount will be charged in the next billing cycle. You get a few days to actually make the payment, but the bill is "generally" generated at the beginning of the billing cycle.
And last night, I received my first bill for the new plan. I signed up for the plan on the 18th.
And it stated that I had to pay $85!!! There was no mention of the $20 discount.