I believe this is the cheapest prepaid plan with international inclusion (on a month to month basis) if you're happy with Optus network
Includes -
Unlimited standard national talk and text
Unlimited standard international calls to 15 selected destinations*
Rollover up to 50GB when you recharge before expiry or when you maintain an active AutoRecharge
*International destinations are: Canada,mainland China, France, Hong Kong, India, Indonesia, Ireland, Japan, Malaysia, New Zealand, Singapore, South Korea, Thailand, United Kingdom and United States
Also, probably one of the few deals recharge every 30-days rather than 28-days (looking at Amaysim and Boost)
$20 belong with unlimited international calls tonnes of GB and not Optus.