There are two main reasons why people with good credit histories/scores get turned down (all over the world):
1. Too many existing cards:
For example, you might have several cards already, with not a single dollar owing on any of them, but with a total of (say) 20,000 NZD credit available. A potential new lender has no way of knowing whether or not you're going to 'max out' all of those existing cards the very next day after they give you a new card, so (to be on the safe side) they treat your application as if you've already got 20,000 NZD of debt.
2. Insufficient income:
It doesn't matter how good your credit score is, if you only earn 50,000 NZD a year, you're not going to get a million NZD of credit. (Remember that it's not the amount of credit that you're currently seeking that's important; it's the TOTAL credit available to you on ALL of your cards that matters, as above).
Chris