This is now even easier to obtain (you used to have to email them and ask nicely)! Simply download the form here, print it and sign it.
I've carried this permit around for the last couple of years, if only because it's a handy bit of almost-ID that shows you are a harmless amateur photographer. In all that time, I've used it officially once - at North Melbourne Railway Station - to make some large format photos. The Metro train staff were completely friendly about it, after I showed it to them and told them what I wanted to photograph... It probably helped that I had the Wista/tripod slung across my shoulder also, because that combo is fairly charming anyway.