Food was not provided and participants were instructed to eat whenever they were hungry, and until they were full.
The diet programme also excluded animal foods, including meat, eggs and dairy, and refined oils, in favour of unlimited amounts of whole grains, legumes, vegetables and fruits.
However, participants in the intervention group did take daily 50mcg doses of vitamin B12.
After 12 months, as well as the weight loss, those who changed their diet decreased their waist circumference by average of 9cm and their medication usage by an average of 29 per cent.
Further, two out of four patients with diabetes reduced their dosage or reliance on diabetes medications, including one who no longer required insulin.
Those without diabetes also reported improved levels of glycated haemoglobin (HbA1c), used to assess a person's control of their condition, with a higher score showing a greater risk of developing diabetes-related complications.
The study's authors are now beginning a second trial to look specifically at the benefits of a plant-based diet for people with type 2 diabetes.
Lead author and Gisborne GP Dr Nicholas Wright was encouraged by the early findings.
"The whole food plant-based approach shows very promising weight loss results that appear to be sustained over time," he said.
"People don't have to worry about going hungry and can still lose weight."
Eating more plants and less animal food was also important for the sustainability of the planet - an aspect of the study that drew support from film director and environmentalist James Cameron and Suzy Amis Cameron.
Elaine Rush, a Professor of Nutrition at Auckland University of Technology, questioned whether the study may have been affected by a small sample group and selection bias - the participants agreed to be part of it - and how accessible the foods would be too poorer families.
Rush also had concerns around the use of B12 and the exclusion of dairy in the intervention, both issues that were important for growing children.
But the study did support the Ministry of Health's guidelines for healthy eating about consuming wholesome foods - and mainly plants.
"There's no one perfect diet - but if it works for these people, that's great because it has made their health better," she said.
"But if we are going to look at translating it into public health messages, I think we have to look a little wider - and even go back to the new Ministry of Health guidelines, which are very sensible."
BROAD study: The findings
• Participants in the intervention group had lost an average 11.5kg at one year, which was greater weight loss than any other trial that did not limit energy intake or mandate regular exercise.
• They also reported an improved self-esteem and quality of life and decreased medications from 94 across the diet group on a normal day to 67.
• One patient out of four with diabetes was able to stop taking their insulin. Two of the four diabetics improved their condition to the point where they no longer met the diagnostic criteria for diabetes. Those without diabetes improved their HbA1c also.
