A dietician is a registered health practitioner who evaluates (or considers) scientific evidence about food and nutrition and then translates it into practical, workable strategies. 

New Zealand dieticians are usually consulted when a patient has a health condition that is directly affected by diet, such as kidney disease, heart disease or diabetes. Dieticians also work in partnership with individuals, whānau, communities and populations to support optimal health and well-being. 

A nutritionist on the other hand focuses on disease prevention and health and well-being promotion in the general population.

A Registered Nutritionist is a trusted authority on nutrition who will provide nutrition advice to the public informed by scientific evidence as well as individual dietary advice by those working in practice. In New Zealand, the term is not protected so it is important to seek the advice of a Registered or Associate Registered Nutritionist. 

You can find a dietitian via the Dietitians' New Zealand website.