Dodo
28GB

At speeds up to 256 Kbps

$20.00/mth
for first 6 months,then $28/mth
Optus

50GB

At speeds up to 150Mbps

35.00/30-days
$35 SIM for $15. Online and new services only.
Felix Mobile
25GB

At speeds up to 150Mbps

$12.50/mth
for first 3 mths, then $25/mth
with code FELIX50

There isn't really a truly unlimited mobile data plan available in Australia right now, though there are plenty that let you do away with excess data charges. These are the providers offering 'endless' or unlimited data in some way, shape, or form. 

Provider
Endless data speed

Felix Mobile

20Mbps

Vodafone

2Mbps to 10Mbps (depending on plan choice)

Telstra

1.5Mbps

Optus

1.5Mbps

Belong1Mbps
Dodo256Kbps

Cheap plans with no bill shock

More data than you need (so pretty much unlimited)

Look, the average Australian uses around 15GB of data per month. You probably don't need unlimited data. BUT if you want an extra buffer, there are plenty of plans offering crazy big data for an affordable price.

These plans have at least 50GB per month.