Are you just looking to transfer funds from one account to another in the same currency, or are you looking to convert from NZD back to GBP or something similar?
If you’re looking to do a conversion, there are plenty of FX money-remitters who will do it at probably a better rate than a bank will give you. Larger transactions will get you a better price and lower transaction costs, so move it all in one chunk if possible.
You can look at prices online, although you will have to register as a client and provide some info to get you through the compliance checks before you can actually transact.
(I work for one such company, albeit on the back office side)