A just-published University of Otago study led by Prof Tony Blakely made the point that one plus one equals more than two for health spending in patients with multiple conditions.
The research showed caring for patients with two or more chronic diseases cost the health system more than it would to treat each disease in isolation.
Prof Blakely, of the university's Wellington campus, is the first author of the study on the health costs generated by individual diseases in patients and the extra costs of treating patients with multiple noncommunicable diseases.
This is one of the first studies to assess the cost of multimorbidity on the public health system, and was published in the journal PLOS Medicine.
The researchers said that internationally there was a "surprising lack" of disease-attributed costing studies that examined multiple diseases in the same patients.
"Governments and health systems managers can improve planning and prioritisation, knowing where the money goes," the study said.
Prof Blakely said Otago researchers had already been having discussions with the Ministry of Health over issues raised in the study.
What stood out was the "high cost of musculoskeletal and neurological diseases" and this was "greater than the priority we give them in planning and health services research".
Costs in the year of death, if dying from chronic kidney disease, musculoskeletal, or diabetes were "particularly prominent", ranging from $13,000 to more than $30,000.
Researchers studied health data for all adult New Zealanders and found 59% of publicly-funded health spending was attributable to non-communicable or chronic diseases.
Almost a quarter (23.8%) of what was spent on caring for patients with two or more diseases was above and beyond what the diseases cost individually, he said.
"If there were no additional costs due to the complexity of having two or more diseases at the same time, we would spend a quarter less on chronic conditions."
New Zealand's high quality public health data meant that our researchers could do analytical work "that many other countries can't," he added in an interview.
The researchers analysed nationally-linked health data for all adult New Zealanders, including all publicly funded hospitalisation, outpatient, medicines, laboratory and primary care over seven years (July 1, 2007- June 30, 2014).
They calculated annual health spending per person and analysed the association of this spending to whether a person had any of six chronic diseases - cancer, cardiovascular disease, diabetes, musculoskeletal, neurological and lung/liver/kidney diseases - or a combination of any of them.
Apart from the "extra" co-morbidity costs, the remaining three-quarters of spending on chronic diseases consisted of heart disease and stroke (18.7%), musculoskeletal disorders (16.2%), neurological conditions (14.4%), cancer (14.1%), lung/liver/kidney disease (7.4%), and diabetes (5.5%).