There are two sizes of tow ball commonly used in New Zealand – the NZ standard 1″7/8ths (47.5 mm) and the 50mm. The 50mm is commonly found on European and American vehicles.
The larger tow ball will not fit a smaller coupling. But, take care not to use the smaller tow ball in a larger coupling; they can come apart. We see it often when people are towing imported caravans and floats.
The tow ball size is stamped on top of the tow ball itself.
Incidentally, the tow rating of the whole tow bar will be on a plate on the tongue of the bar. If not, talk to a tow bar manufacturer who will be able to point you in the right direction. Without the tow rating on the tow bar, you may also fail a WOF. To determine towing capacity of the tow vehicle, you need to take note of the manufacturers specs for towing as well as the rating of the tow bar.