Not the Waimak, but interestingly the technical mouth of the Wairau river is not at the Wairau Bar itself, its a couple of km’s further inland. I do not know the exact location myself, but was told that by one of the councils river engineers.
So anyone fishing at the Bar could probably legally keep any trout or salmon they might catch. There are two catches, one is a little tidal marsh about 500m upstream, if that is classified as a freshwater source entering the Lagoons then the Bar would just be inside its radius. The other catch, is wither the Lagoons are classified as a saltwater or freshwater system.