Cast nets are legal in NZ.

From the MFish website

Can I use a casting net in New Zealand?

There are no specific restrictions against using casting nets in New Zealand, however there may be closed areas (rähui), marine reserves or mätaitai to consider when setting your net.

The size of the mesh depends on your targeted species. For bait fish such as herrings, pilchards and pipers etc the mesh size is 25mm.

Regulations and mesh sizes for fishing nets are shown in our regulation brochures, which you can access here: Recreational section

So if you getany other fish in your 25mm bait net you would have to release them.