You decide how hard the assessment day will be. It's all in your head. If you go in there in a positive, enthusiastic and genuine mindset your body language and your speech will reflect that and you'll breeze through it. It's okay to be nervous and no one will judge you for being nervy - but for the love of God, be positive and enthusiastic.
A lot of people react to stress and/or nervousness with defensiveness and that is what kills them. They sound like deadbeats, their body language goes to shit and they're toast. And a side note: if you get defensive with the psychologist you're going to have a hard time.
You asked about what you should know and my answer is just use common sense. They aren't going to expect you to be able to rattle off answers about the ADF like rain man and they aren't going to expect you to know specific insight about the job because the truth is you haven't been there and you haven't done the job yet.
Just know everything on your job page on defencejobs. Know where your training is, how long it goes for, what you learn and realise the challenges you are going to face. Learn as much as you can about the ADF and develop a picture about the basics.
They ultimately care way more about your character and your life experience than anything else. You mentioned you've done scouts and you play sport and you are aware about what motivates you. They'll love that. That's 95% of the work done. The rest of the job is just common sense and positive interaction.
One little thing I will add down the bottom is the psychologist. The psychologist can sink you. They can make a judgement call at any time to end your process. Good psychologists get answers out of you without even asking questions, and that is what mine did. It felt like I had a 20 minute conversation with her and I left scratching my head because she didn't even ask specific questions. It was a free flowing conversation and she learnt about my childhood, my parents, my attitude, my morals and my past experience. She was a wizard. Thank God I was positive and didn't give her any concern - but I can see how someone could have let slip something they shouldn't have. Don't treat it like an interrogation - be genuine and project a good image of yourself and you'll be fine.