From fish and game website
Kahawai Fishing - Do I need a sports fishing licence when fishing at the river mouth? | North Canterbury
"Fish & Game are aware that there is some confusion out there amongst anglers in regard to whether or not they require a sports fishing licence to target kahawai at river mouths. The legal position is:
If you are fishing in fresh water and using gear that could reasonably be expected to catch sports fish (Trout, Salmon and Perch) you need a valid sports fish licence. Freshwater is defined in the Conservation Act 1987 as any river, stream, lake, pond, lagoon, wetland or any other body whether naturally occurring or man-made. Freshwater also includes the mouths of rivers and 500m along the seashore from where the river meets the sea at low tide).
So if you are apprehended at a river mouth fishing with gear that could likely capture sports fish (bearing in mind salmon will take large silver kahawai lures), there is a legal requirement for you (not Fish & Game as it is a strict liability offence) to demonstrate to a judge that the gear you were using was not likely to capture a sports fish such as a salmon."