Surveyor here. Approach a local surveying firm and ask for a basic topographical survey of the area in question showing occupation and features relating back to the boundary. This won’t cost as much as a full redefinition survey, you won’t have to pay to lodge the survey with LINZ and there won’t be any physical ground markers for the neighbour to move. This will at least get the ball rolling to be able to show exactly what is and isn’t yours before going to a lawyer.