Recontract with Optus and get $10/month off selected Optus Choice plans for up to 24 months, for post-paid mobile services. Amount of discount appears to vary from account to account.
You will need to check your Optus-registered email address for the link, to redeem this offer, as it contains the unique link.
So it's highly likely that it's a targeted offer (just received my offer email).
I also received it. I can get $1 my monthly spend but on a 12 month contract.