I just spoke to Telstra, and they can move me to a $79 cap! So I've signed up. I have to make Mobile Phone Repayments, (of $62/month) and also pay a plan fee of $79/month, but for this, I get $450(+100 to other Telstra Phones) of calls.
This beats the pants off last time I tried to do it when they told me that I'd need to cancel my existing contract (at a cost of $800) to move onto the cap.
Huzzah for credit!