mizuki
I did the Melb Foundation test last year for St Caths and made it to first round interviews. The process has you do an online test I remember and you have to be in the top 20 to get into first round. I think last year they had 300~ applicants and invited 20 to the first round. The first round consisted of a maths testing and two interviews (they ask "tell me about yourself", "how many countries have you been to?", "how do you plan to travel to school if you get in").
The actual online test was for me not too difficult and it was on like a google form. It was not much difficult compared to the selective exam. They're offering a full scholarship and I remember that is was run by the older students who got the interview from the year before. It's a super good opportunity and unfortunately I can't tell you more about the second stage when they invite you and your parents for another interview bc I didn't make it that far.
Make sure you read through the whole school's website, look for all opportunities they offer that your current school doesn't but don't talk bad about your school. They'll ask what school you plan to go to next year if not to the Scholarship. Just compile a good list of academic and extracurricular achievements. And they ask of books you've read that outside of your year level.
Best of luck!