Hi Hayley!
There are a few options you have regarding working for the ATO. The most obvious option is to complete a business or commerce degree with an accounting major. Pretty much every university offers this, so choosing the one you prefer wouldn't be an issue.
Secondly, if you would like to incorporate more of a law aspect into your studies and future work, you could either do a Law degree, or a combined commerce and law degree which would allow you to be both a a tonne of graduate positions for students who've done any one of the options above. If you wanted to specialise in taxation as you've mentioned, you could choose that field as you pathway.
I believe UNSW also offers Taxation as a major for a commerce degree (as opposed to accounting). I would keep in mind though that specialising in taxation before you've even studied accounting at all probably isn't the best choice. I know a lot of the people at my work like tax, but it's very much the only thing they can do which makes moving to different paths a lot harder. ( Good thing people always need their tax done then!!)
Back on track, with working in accounting you can also specialise yourself in more legal based pathways down the line if that is what you wanted to do. You won't necessarily need a law degree to work in legal related lines. For example when I worked in insolvency, it had a large component which revolved around the law.
As for working for the ATO you have two options; Apply as a graduate to their program and enter that way, or work elsewhere and then apply when you have more experience. I must admit from my own research the second option usually seems like the better one. A lot of the people who answer the phones at the ATO are the grad kids, and I assure you that isn't a fun job! Building up a little more experience elsewhere isn't a bad thing.
Finally, not sure if you've seen the Commerce/Business section of this forum but there is a stack of stuff for people looking to pursue accounting; Plenty of information about things like cadetships and graduate roles, and the mods are a very helpful bunch if you have any questions.
Hopefully that helps you out a bit, and best of luck with your future.
(Ps. Not sure if you've ever been to the ATO office at world square in the city but it's litterally beautiful and I wanted to live there the first time I went
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