Unfortunately, I think you can get away with doing very little actual mathematics beyond high school level, hence the masses of teachers of subpar quality. (I once sat in a course with some people training to become mathematics teachers at ACU, and the quality of the students seemed fairly shocking.)
If you want to be a GOOD teacher, particularly of the extension courses, I would recommend a solid undergrad course like a BSc with many mathematics courses at a good university in addition to the DipEd.
(I stress that I am not sure of the exact qualifications required to be certified to teach in NSW, I'm just speaking of the level of understanding I think that any mathematics teacher of courses higher than general should aspire to have in order to best benefit his/her ability to teach.)