Disclaimer: This is not financial advice. You are responsible for doing your own research before investing.
It can be difficult to know which broker is cheapest since the pricing structures are complex. This analysis attempts to figure out the total costs for a simple investor depositing a fixed amount of NZD every month, investing it using equal sized trades and then withdrawing everything after some number of years. Rate of return is assumed to be 7% annually for all investments. You can use a back-tester like Testfol.io to a more realistic idea of expected returns.