Depends on your age. 18-21 (I think) get paid at award rates and 21+ get paid at higher rates. Since you're casual, you get the extra 15% loading and depending on the times you work you may get extra. For eg. I worked at one of the hubs where the only shift you can work is 3pm-8pm. Since part of the shift falls after 6pm, they have to pay you a loading of 15% on top of the casual loading, so overall I got around $22ph. If you work in the mail centres (Strathfield/Rockwood and I think Alexandria still) then you pretty much work a certain 5-6hr shift which is chosen from pretty much anytime of day, so like could be 9am-2pm or even 1am-6am. You won't get extra loading between 8am-6pm, you get like 15% extra between 6pm and 11pm or something, and an extra 30% from 11pm to 8am. Some people like working in the wee hours of the morning, so yeh.
It's usually 5 days a week for about 4 weeks and you shifts are 5-6 hours a day depending on where you work. So if you do the maths, it's pretty good money for only a few weeks worth of work.
As for applying, I'm not sure if they are still taking applications. Try calling the Aust Post head office (number in white pages) and ask them.
Hubs: This is where I worked. You pretty much just sort mail and put them into boxes. Kinda boring, but there's a number of hubs around Sydney (like Seven Hills, St Marys, Wetheril Pk, St Leonards, etc) so you can work closer to home.
Mail Centres: These are the huge mail centres that process mail, located at Rockwood (near Strathfield) and Alexandria (I think it's still there). You can do a number of tasks here, like sort mail, use the barcode
printing machines, use various other machines too. You wouldn't use many machines (if any) at the hubs.
There's also the new parcel and international mail centre at Clyde which seems to have more interesting jobs, except with parcels and international mail, so you have to quarantine mail and stuff. Similar pay rates to above.