I did some research, and it looks like there are (at least) three major carriers that offer affordable pre-paid SIM cards with data plans: Virgin Mobile Vodafone Telstra
travel.stackexchange.com/users/1161/offby1
I don't have enough rep to upvote Enchanter Thunderbirds answer , nor comment, but I did the calcs and Woolworths Mobile prepaid sims
( http://www.woolworthsmobile.com.au ) are the best value as at April 2013.
You end up with the same talk time as amaysim which is the other cheapo, but you get more data.
Way more data.
5GB.
I was there for a month and had over 4GB left.
The sim is $2 and the top up is $29 ish.
You can get them from any Woolworths supermarket or the smaller ones (used to be Bi LO shops).
Easy peasy.
The Woolworths sims also use the Optus network which isn't too bad even in regional areas
(I was at Scott's Head, NSW for 2 weeks and had no issues).
travel.stackexchange.com/users/6513/Scotty.NET
savvymax and lebara-mobile are two smaller providers.
lebara-mobile is good for calling overseas, savvymax credit does not expire.
they don't work in the outback.
travel.stackexchange.com/users/1619/Amelse Etomer
2 years ago, I got a Vodafone SIM card with a prepaid 1GB / 1 month plan and a SIM-locked 3G USB stick for 100 AUD.
There's probably a similar plan available without the hardware for less money.
I got it in a phone store in downtown Sydney, not sure about the airport.
Reception varied a lot, of course; I sometimes had to make do with GPRS speed, but didn't get a signal only where one would expect that (i.e. in the outback or national parks).
Note that activating the card had me going through a fairly complex online registration process
(IIRC you could use the card itself for that) which required a valid Australian post address (I used the hotel's without any problems).
travel.stackexchange.com/users/1371/Michael Borgwardt
As was mentioned already the major two carriers in Australia are Vodafone and Telstra.
Also there are two smaller ones like Virgin and Optus.
While the smaller carriers offer some times cheap plans, their coverage is lacking.
When I was in Australia in October 2011 I decided to go with Telstra.
I don't remember exactly why, but maybe because they were cheaper.
Data plan was an important factor for me
and I was very happy with their service.
I bought the sim for 30AUD which you get back as balance on your account.
For the same amout I bought a plan that included data & call minutes.
The plan was similar to what they call now Pre Paid Cap Encore, which means you get $250 credit calls (which run out quick because of high minute tariff) and 500MB of data.
You can see additional details here: http://www.telstra.com.au/mobile/prepaid/prepaid-offers/ Don't be tempted to buy the sim at the airport from Vodafone -- at the city they cost less.
travel.stackexchange.com/users/874/Arik Fraimovich
I am using Woolworth mobile.
It gives you $250 credit, another $250 credit for optus mobile only, 5G Data and 45 days expiry.
You can check it by yourself.
http://www.woolworthmobile.com.au
travel.stackexchange.com/users/3952/Enze Chi
I'm arriving in Sydney on Friday for a week.  Ideally I'd like a pre-pay simcard for text and data (I need to check my stackexchange sites of course ;)).  However I have no idea of what companies or options exist, and what would be the best for a traveller in terms of moderate data usage.  Also, is it possible to buy these at the airport on arrival?
Prepay simcard for data in Australia
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<australia><cellphones><data-plans><pre-pay>
travel.stackexchange.com/questions/4712
travel.stackexchange.com/users/101/Mark Mayo
The three main carriers with affordable pre-paid SIM cards with data plans are Virgin Mobile, Vodafone and Telstra. Woolworth mobile, savvymax and lebara-mobile are three smaller providers; lebara-mobile is good for calling overseas, savvymax credit does not expire. While they somtimes offer cheap plans, their coverage is lacking. Major mobile providers are Vodafone, Virgin Mobile and Telstra. There are, in addition, smaller providers, such as lebara and savvymax, that may have cheaper offers. However, their coverage is lacking. A final service provider is Woolworth.
The major mobile carriers in Australia are Vodafone, Virgin Mobile and Telstra. Smaller mobile providers include lebara and savvymax. They are sometimes cheaper but coverage is lacking. There is also a Woolworth's sim.