Optus 3G or 4G network. Shopped around and this seems like the best plan for low-variable data and mobile usage as I am looking to move away from Amaysim which has gotten costly.
Before the discount, the plan total cost is $20 (SIM) + $5 (postage) + $18-$36 per month.
May 2015 deal. Last month offered free Activation/SIM postage which was roughly of equal value (save $25).
The plan cost per month varies from $18 to $36 and can be tailored to individual usage. Ramps up to unlimited voice/text and 2.5GB.
The best approach seems to be to default to the minimum $18/month and have it bump up as usage requires, as there is no penalty for going over your imposed limit - you are simply upgraded to a higher usage need (see the link).
The exception to this is data above 2.5GB which is charged at $15 per excess GB so if you require data above this then other larger unlimited plan providers may edge this deal out.
No personal experience yet, but opinions seem good to reasonable.
You can also get additional savings by using the referral links below:
https://www.ozbargain.com.au/wiki/spintel_referral_links
Further details:
https://www.spintel.net.au/referafriend
$5 postage fee