You have to prove in some
official way that you have English ability. If the course for your degree was taught ENTIRELY in English, you would have to write to the university and ask for a letter stating that you (name), date of birth (whatever), attended the course for (whatever degree) between this date and that date, and the course was taught in English, then that letter would be proof you could send to INZ. Or if you have a job where you have to communicate in English in speech and writing at a high level of fluency for more than half your time, a letter from the employer saying that could be evidence. Without something like that, you have to sit IELTS and get at least a 6.5 average.