I did Biology and Chemistry in Year12. I did the lowest English (am science, maths person not an english history person). I started Nursing and once I was a registered Nurse then went on and did my midwifery. I wouldn't describe myself as the brightest tool in the shed. I was very very average but as I have gotton older I think I would do so much better if I went back to school. My intentions were always to do Nursing only to do midwifery. When I started training to be a midwife you had to be a registered nurse where as now some places do offer dirct entry midwifery which means you don't have to do nursing first.
I would seriously consider doing a science subject but stick to an easy one like Biology and avoid Physics and Chemistry as they are much harder. Although they are pre-requirements for uni its not essential and you can do bridging course when you start uni to catch up so to speak so not doing a science subject wont ruin your chances of getting a placement.
There are a number of ways you can do midwifery depending on how quick you want to do it. Also you don't always have to have Yr12 to get into uni. I am not sure what mature age is now. I think its come down the age it might be 20 now not 21.
Is there a uni near you that offers the dirct entry midwifery? I don't know much about this but I do know most places do not offer this. They might offer it with nursing as a double degree.
If your after a way to earn money whilst your studying then enrolled Nursing would be one option as you get paid to do it, you cement the foundations of nursing however after your qualified you would then have to do registered nursing and then midwifery so its the long way around so to speak. The other options would be to start your bachelor of nursing at uni and once finished then do midwifery or do the double degree that would mean you would be a midwife and a registered nurse once you finish or do the direct entry midwifery which is a harder course to get into and it might mean moving depending on where you live to study it.
Best of luck if thats what you want to do you will get there.