The reason they have a higher population density is because they have more people on average in a certain area, because New Zealand is smaller they are given less space to live, this means that more people are crammed together, making the density of people within a square mile higher. When you give less square miles, you reduce the amount you have to divide the population by, which makes the density go up.
( In case you wanted the calculations for the density on New Zealand and Australia, it is about 41 to 42 people a square mile in Zealand, and maybe 7 or 8 per square mile in Australia.)
Hope this helps, if not, comment below please!!!
