MG, another secondary teacher here. I completed a degree in science first, then went back a little later and did a graduate diploma of education.

I am qualified to teach secondary maths (at all levels) and science, and the senior science subjects I'm qualified to teach include physics and chemistry.

I'm not sure if there are different rules in different states, but as I had a science degree, junior science was a given. I had to have completed a certain number of credit points in mathematics subjects to get the maths qualification as a minor teaching method (ironically, science may be my major teaching method but I only teach maths now). For the senior science subjects it came down to which areas I had studied at uni. I had a major in chemistry so could tick off that one easily. I only did first year physics at uni, but that was enough to satisfy the NSW department of education that I could be a competent physics teacher, so it was also ticked off.

Qualifying to teach specific secondary subjects comes down to what sort of degree you have done, and which subjects were included in that degree. Perhaps contact the education department at a university and talk to them about what teaching methods your degree allows you to teach with a graduate diploma or whether you would need to study another degree altogether.

BW