Try the Turbocharge option on Optus Pre-Paid or Boost Pre-Paid.

$25 recharge gives you $100 credit (Boost only, not Optus)
$30 recharge gives you $120 credit
$50 recharge gives you $250 credit

Calls are charged at 37c/30 sec block (versus Vodafone's 30c/30sec, billed
per sec)
Also flagfall is 25c (versus Vodafone's 20c)
But those shouldn't matter to you much if you're not using up your credit!

The benefits of Turbocharge for *you* are:
- Rollover.  If you recharge before the credit expires (30 days), your
unused credit rolls over (ie remains valid for another 30 days).
- Low minimum monthly spend.  Depends on how close to $230 you're spending
now, though.  Your best option might even be the $50 to begin with, then
just recharge with a lower amount next time if need be.
- Free SMS.  You get 4 free SMS to use to any Australian mobile network,
every day.

http://www.optus.com.au/turbo

Stop using your Cap and go back to normal pay-by-the-minute prepaid