There is no justification for paying one teacher more than another based on the subjects each teaches because each does the same job. If maths and science teachers end up being paid more than English and history teachers, the latter would quite justifiably reduce their workload and expect the former to pick it up. If the maths and science teachers ended up with fewer classes to teach as well, other teachers would be rightly outraged.

That is not the same as paying teachers more if they have additional qualifications because one would expect the additional qualifications to result in better student learning, and systems used to recognise additional qualifications in pay scales.

Teacher registration should carry from one state to another via a mutual recognition system of the various state registration authorities. This will happen, though it should have happened already.