Understand the purpose of the department, we are here to serve those who served, and then truly consider what you would bring to this department. The department deals with current and ex-military, and discipline and respect is highly valued.
Don't over think about what the Department does (definitely look into it!) but think about why you want to work for it, and what you see yourself doing in 5-10 years time.
Read up on veteran legislation and policies; don't go overboard, but know some of the things that DVA is doing with regards to commemorations, mental health, etc. And if you have a basic understanding of military culture, so much the better!
Before they come to interview know about the department and company and based on that you can decide which one is best to you.
Train yourself on how to answer application and interview questions with the STAR method. Look at the APS Levels 1-6 to see which types of examples you'll need prepared for your interview
Note: Interview processes vary year by year.