I used Optus 4G Social $50 tariff when I was in Sydney for 3 weeks over Christmas. The tariff comes with 2.5GB of data and unlimited social data (Facebook, my space, twitter, etc). In three weeks I used only about 500MB because of the unlimited social data.

https://www.optus.com.au/shop/prepai.../prepaidoffers

I bought it at an Optus reseller and had to show passport and a credit card. When I picked up a second SIM card from another dealer, I didn't get asked for any ID.

Speed was pretty decent. I was able to achieve LTE speeds around the CBD and in the inner suburbs. At one point, I hit 60Mb/s