I have had this problem with the languages that I have studied. This is what I do: I watch many hours of video clips of talk show interviews from the country whose accent I want to understand. In America, this would be Conan O'Brien or Jimmy Fallon, but in New Zealand I am sure they have other talk show hosts. This format is perfect, because they speak like normal people and quite fast. They use a lot of current slang words. And the questions are almost always the same, which makes it easier to figure out what they are talking about. Good luck!