Do I need to exchange my NZD before investing
Yes, if you have a cash account, you will need to exchange the currency into the currency of the stock you are buying before you trade, e.g. if you hold NZD in your account and want to buy a US stock, you will need to exchange NZD to USD in the app before executing the trade.
If you have a margin account, you have the option to trade without exchanging the currency, but you will be charged interest. For more information on interest charges, please refer to the link.