While this is a bit OT, obesity is an increasingly large problem for much of the developed world. Especially for countries/regions where the average activities are becoming more sedentary, which is not just a Kiwi problem.
Also yes, BMI scores can be used as a tool to determine obesity, it is just not a very accurate one. A short, very muscular weightlifter/body-builder can fairly easily have a BMI indicating obesity or overweight, yet have a very low BFP (body fat percentage).
BMI is reasonably decent tool for studies of populations, but not for individuals, because it is based on averages. When looking at individuals, it is better to measure their BFP, and/or waist circumference to height ratio.