Alex,I had that same question as you. The feedback that i received was that one of the reasons for interview was to ascertain the communication skills in english.It is not mandatory however. Neverthless i appeared for IELTS exams and sent them my scores while sending my application. The exams typically are half a day affairs, they have 4 areas for testing and they give individual scores for each and a composite score. The scores go from 1 to 9,
9 is the highest score and i guess a score of 5 or 6, they consider as a average.I believe that for emigrating to New Zealand & Australia IELTS is mandatory. I hope that helps.
sunil