If you upgraded the plan mid bill cycle you will have both available data wise (only charged via pro rata)
Just a "quirk" of the system that it pro rata the billing charges but not the usage.
If you did not upgrade mid month then its something entirely different and you need to contact Optus about that (e.g. webchat etc)
Hey @Rossj, your available data usage from the old plan likely rolling over into the new plan for the current billing cycle. We can take a look into this for you, just send through a PM with the full name, account/phone number and DOB attached to the service and we'll work from there.