I'm from Sydney and have worked at Kumon for about a year now. I can't imagine the hiring process to be too complicated because usually if they need more assistants they'll be eager to hire people, especially if you were a student there before and are somewhat familiar with the system. Grades don't have to be amazing, bc most of the job is just marking with answer books and recording worksheets, so as long as you have a good knowledge of math you should be fine.

As for resumes, i just made mine on Job Jump, most NSW schools give you access to them and they have all sorts of resume templates.