Yes you can mention. Be clear and honest. You may or may not justify backlogs, depending on whether your reasoning weakens or strengthens your position. If you have a good convincing reason, then must do so. But you should not unnecessarily worry about a backlog. For your knowledge, A good IELTS score plays a crucial role for students who have backlogs and would want to study in New Zealand. If you have an IELTS band of 6.5 and have enough research projects or extra-curricular activities listed in your essay, chances are you can back up your backlogs with these academic qualities. When you apply with backlogs at universities in New Zealand, make sure you are aware of all the norms of each university. Good Luck.