Have a look at vaya mobile (optus reseller). They have a very good BYO mobile plan deals. The most expensive one is only cost $29/mths and that is included unlimited calls and texts (13,1300 and 1800 are included in the plan), 4GB data, $100 international call and tethering is available.
So if you buy 2x of this plan, it will cost you less than $60/mths or one of you can get the cheaper plan
I second Vaya. I have moved all of us over and for 4 of us we are paying $32 a month!!!
The boys are on a $7 a month plan as is DH and I have the $11 a month plan. I use my phone for texting, calling and internet and don't come anywhere near the limits.
Try whistleout.com.au - they can show you the cheapest sim only offers.