At 100 GB you should probably look to changing your download habits slightly as staying under that amount should be pretty easy.
IMO $80 for 100 GB is already a very good deal. Usually the only way to increase that is pick a similar plan with higher allowance. Current special deals don't apply. You'll need to contact Optus directly on live chat to determine if that possibility applied to your plan.
Otherwise if you get a new phone (s9 256gb?) from Optus on a plan then it could be $135 for 100 GB. Use the sim on your current phone and (easy) sell the new phone for say $840 (brand new in box) on eBay and you've got your $100 for 100Gb plan. Chip in $10 more a month and you've got 200gb a month