Lyca mobile have cheap rates as do Amaysim
You can buy the sim cards from most places as long as your phones are unlocked.
http://www.amaysim.com.au/
http://www.lycamobile.com.au/en/
Plus the added bonus of calling internationally too
We are on optus networks and can call each other for free but agree that the pre-paid expiry sucks!
There are apps that you can use too but they use wifi or need data for it to work - ie WhatsApp, Viber, Skype
Not sure if this helps you in anyway.