''It gives the first accurate picture of the burden of the disease in New Zealand,''study lead author Prof David Murdoch said.
The study also had ''international implications as few places routinely test for the potentially deadly - and preventable - bacteria'' he said.
The southern health region, comprising Otago and Southland, had the second-lowest rate of detected Legionnaires' disease, ranging from 2.0 to 3.9 cases per 100,000 nationally in the first national study of the disease burden.
This was well below the new, higher national average of 5.4 national cases per 100,000 New Zealanders, which emerged from the study, undertaken in 17 of the country's 20 health boards.
Canterbury had rates of between 6 and 8 cases per 100,000, having also previously recorded reasonably high numbers of cases.
Prof Murdoch, a clinical microbiologist and researcher at the University of Otago's Christchurch campus, said some parts of the country had ''already adopted the more intensive form of testing'' used for the first time throughout New Zealand in the research.
To get their results, the research team tested all respiratory samples from patients with pneumonia admitted to 20 participating hospitals, including Dunedin Hospital, between May 2015 and May 2016.
During the year a total of 238 cases were identified, or 5.4 cases per 100,000 New Zealanders.
Of the 238 cases, 15 died within 90 days of diagnosis and 38 cases required treatment in an intensive care unit.
The study has just been published in the international medical journal Lancet Infectious Diseases.
Prof Murdoch said that international publication of the findings was getting Legionnaires' disease ''on the radar as more important than we thought in terms of numbers''.
The study of the burden of the disease in New Zealand showed the incidence was ''much higher than previously identified'', and was ''the highest reported incidence in the world''.
More accurate testing of patients admitted to hospital with pneumonia meant they would benefit from faster access to the different, specific antibiotic treatment required to counter Legionnaire's disease, rather than being administered broad spectrum antibiotics which could help foster antibiotic resistance, he said.
The study identified the most common form of bacteria causing the disease in New Zealanders was Legionella longbeachae (in 63% of cases).
This strain is found in soil and composted plant material, and people at greatest risk are those involved in gardening activities, including handling compost.
About two-thirds of cases were admitted to hospital during winter and spring, and almost 60% of patients were aged over 65.
