Hi @explor thank you for using the plugin!
You can improve this by adjusting the height of the map. Since NZ is predominately vertical, you can make the height much bigger and it will adjust the space on the sides.
You will still some space above and below, mostly because the map includes the smaller islands, like Raoul Island on the north and Campbell island on the south, so it leaves a lot of space on the top and bottom to accommodate them.
In the Pro version you have more granular control, but in the free version we can only adjust the height.
Greetings, Carlos