You won't catch crayfish in nets (unless bobbing for it).
You'd need a craypot and drop it off a boat, the person selling you the pot will be able to give you advice on how to set it up.
Freediving for cray is a different story… Rocky coastlines, a wetsuit, good gloves and patience is a start. Google some youtube videos how to catch them. Small ones are a no go as are female in berry and soft crays.
Here's dem rules:
http://www.mpi.govt.nz/document-vault/936