Just on the Optus website
The month to month option is there. Usually you would arrange not to get the modem sent out by contacting customer service. But that might not be possible today. I'd order the deal as is and pay the $200 for the modem and not open the box. Arrange to send it back for a credit when you can get through later.
Worst case though is you have paid $200 extra you didn't need to.