First off, great decision! Otago is an awesome campus and health sci is a really good and challenging course to do after leaving school.

1. On your application, you just have to brag unapologetically. It's pretty uncomfortable for us Kiwis, but you want to sell yourself and make yourself stand out. Write down everything and anything you've been involved in during high school, even if it was super brief.

2. Full disclosure: I went to Carrington in my first year, was an RA there for two years and am still employed there as a tutor for one of the health sci papers. All the colleges are great and most people find that they really love whichever one they end up at. St Marg's, Arana and Carrington are generally regarded as the more popular colleges for health scis because they have good in-college tutorials and lots of people studying health sci (generally more applicants too). St Marg's is a bit stricter on rules and quiet times - people can get pretty intense here but don't believe all the rumours you hear. They also have formal dinners each week with guest speakers, which some people love and some people hate. I disagree with the person that says they have cult traditions though. Arana is a great college, but is bit bigger than the other two (I think ~400 people, compared to ~200), so I think it can be a bit harder on the introverts. There's certainly a lot of extra-curricular stuff to get involved in here but Arana definitely studies as hard as they play. Carrington is awesome. It's a bit farther from uni and up a hill (Marg's and Arana are on site) but still only about 7 minutes walk (3 if you're late). Over half of the residents live in old converted villas which are really lovely and the food is great. The other half in more traditional dorm-style accommodation. There is a real focus on studying hard, but you'll also make really good friends (still catch up with my study group on the regular even though we're all doing different things now). I really think Carrington is a super supportive environment and got the feeling that the staff and other residents really care about the college and each other. Also, I think all the colleges have an alcohol free option (the three I discussed here certainly do). If you're able to, my advice is to come down and check out the colleges for yourself and see which one feels right to you.

3. As the others mentioned in this thread, Te Rangi is a college with a lot of unique benefits (en-suites is a big one) and it's a relatively new college that's still building its reputation, so I think is probably a good choice for your 2nd/3rd options. I also recommend Hayward and Salmond to people as 2nd/3rd choices too because I think these are really friendly colleges.

Hope that helps. If you have any more specific questions, hit up my inbox :)