Hello! I'm almost the same in terms of the going into a secondary education-type career - I'd like to teach maths though
From my own research, Maths/Science teachers are in huge demand. Articles in the paper and what not say that there is a mass undersupply of Maths and/or Science teachers nation wide, where there is a mass oversupply of everything else, particularly English/History. From interviewing my own teachers, I guess it would be beneficial to yourself having done Ext I and Ext II maths, but you only really
need 2unit to become a maths teacher. If your results are good, you present yourself well, work experience and all that, then it is very likely that you'll get a job sooner rather than later =]
I'm not so sure about what science is like since I'm focusing more on the maths, but I think doing an educational degree with science subjects (specialising in the subjects that you want to teach, for example Chem/Physics) would be fine for you to go out and teach. However, people who are specialised in Maths and Science teach in the areas that they want to teach - so either or, since both are in such short supply. If you were to teach, for example, Geography and Maths (with Geo as your main major), then you would be able to teach senior Geo but would also get junior maths classes at a regular school.
Hope I've helped! I only encourage you to do more reading/research, and see what the job market is like, what your goals are (in terms of where you would want to work, etc.,) and get in touch with your teachers to talk more about what the career is all about. Work experience always helps! Good luck =]