What happens after your flash sale 12month contract ends? For example there is a 80GB for $40 flash sale happening as of 22/4/19. Usually it's $50. After 12 months, if I wish to stay with the plan, does it go back up to $50, or does it stay at $40 until I decide to change my plan?
It will continue at the same price of $40 until your 12 month contract finishes. After the contract is finished they may change the price as stated under the terms and conditions on the link below. Personally I doubt they would put the price up as you are in a position no longer under contract to tell them to get stuffed.
Optus used to just roll any contract plan into a month to month plan that was identical. More recently a lot of plans have now phrased the deal as 'bonus' data which is a bonus only for the life of the plan. This seems to be an attempt to boost profits via the lazy tax (i.e. many people just won't notice and pay the increased amounts) Its unclear just how much Optus are reverting plans or letting them stand as per usual. If they do there are a few options available as @Yeldarb says you are not on contract anymore.
1) Set a reminder in your online calender to check if the price bumps up in 12 months (esp. if you direct debit)
2) 1-3 months before the deal expires start keeping an eye out for a new Optus deal as you can usually jump to that by then.
3) Call Optus and complain - its a fair bet they'd just restore the 'bonus' on request