You can download the form (48R or 48RTha) from
Australia Visa Services. I've had my GF here twice in the last year. I went with her to lodge the application in BKK and I think that helps. Take your passport as they want a copy of it. Be there when the doors open, otherwise, you could be sitting around for a while. It's next to Surasak BTS so is easy to get to. Very important to be honest on the application.
Here is a copy (edited to protect the innocent) of my attachment to 48R which details the additional documents that I supplied and why. Also an info sheet from the above site.
Make sure that it is obvious she has a reason to go back to Thailand (a job, but not in a bar, family ties etc.)
Good luck.