You and everyone else. Unfortunately Optus do not provide this option to post paid customers. All Optus customers are potentially on the hook for an extra $1500 each month and in fact the liability is notionally unlimited as Optus retains the ability to charge customers endlessly at their discretion:
If you use more than 150GB on a single billing account we may continue to charge you at the same rates or restrict your data until next billing period.
The only way to limit this exposure is to either switch to a prepaid plan (Optus will shut those off the second you exceed the limit by a kB) or to use a different provider. Other mobile providers have recently switched to 'unlimited shaping' so hopefully Optus will also soon end this unwanted skim that hits an unfortunate many every month and adds much more to Optus bottom line per Gb than any reasonable contract revenue.
In practice I suspect this would be covered under the Unfair Contract Term laws. But someone would have to challange Optus on that first.
Customers do have the ability to monitor and control their data usage by viewing their usage online/app and receive usage alerts.
Also if you have an Android phone you can usually set up a local data usage warning and cut off in the phone settings. This is very reliable as it counts exactly how much data your phone has used. There's also usually a monthly cycle option so it resets to 0 as needed.