Hi,
My dad's currently in recruitment and I'm in contact with the commander of the legal division down in Canberra. If you need any in depth information I'm sure I can get it for you.
Joining in the Legal Officer PQ (professional qualification) is done as you would for any other officer position - see the defencejobs website. The only additional requirement is that you have a law degree/be eligible for admission to practice before they'd consider you.
If you want to get the Navy to pay your HECS and textbooks etc, you will have to go through the sponsored undergraduate scheme (again, info on the defencejobs website). If you join only after completing your law degree, they wont pay off your debt (although you still get a boatload of benefits).
Remember however that the government never gives anything away for free, so the Navy will put you under a Return of Service Obligation that requires you to stay in the Navy from between 3-5 years, depending on how much of your debt they end up picking up. Speaking as someone who changed their mind as to what they want to do about 5 times in the last 6 months of their degree, this is a massive factor worth considering. Make sure it's what you want to do.
Also remember that the ADF can be erratic as to when and where you get posted. You
do get preferences, but it can still be annoying - especially if you're trying to settle down.
As to the kind of work you'd be doing, as a newly recruited LEGALO they'd put you on track to earning a masters in defence law (I forget the exact title). It's completed over the course of about 6 years, in which time you are exposed to a great number of real world and defence-specific legal issues. Certainly a lot of fun! I'd recommend speaking to a recruiter, although when it comes to specific PQs such as this they can only offer a limited overview. They could possibly refer you to a current LEGALO though if you asked nicely