Your credit score is a summary statistic calculated from your credit file: your repayment history, credit accounts and limits, enquiries, defaults and public records. In NZ, three bureaus each keep a file and a score on you, Centrix, Equifax and illion, mostly on scales that run to 1,000, and lenders check one or more when you apply.
The score is a shorthand, not the decision. Lenders read the file behind it: a paid default from years ago with clean history since reads very differently from fresh missed payments, even at the same score. Higher scores open more of the market at sharper rates; lower scores narrow the field and raise the price.
What moves it most is repayment history, balances relative to limits, and how often you apply for credit. What does not touch it: your income, your savings, and checking your own file, which is free with each bureau and worth doing yearly.
The bands and the mechanics are covered in what is a good credit score in NZ, and the fastest honest ways to lift one are in how to improve your credit score.