People with intellectual disabilities are dying 20 years before other New Zealanders, and no one knows why.
This is the core finding of a report that cites a critical lack of information about these deaths.
Author and disability researcher Sharon Brandford analysed official data and found "a piecemeal and uncoordinated approach" to avoiding premature deaths among this group".
"Most of the agencies that do look into mortality don't have easy ways to record and flag that the person who died was intellectually disabled. And that's mainly because they don't die from having intellectual disabilities."
She said it renders the deaths of people with intellectual disabilities "largely invisible".
"The difficulty for people with intellectual disability is they haven't really been counted clearly, and efforts to try and address poor health outcomes and low life expectancy can't be measured, because we aren't tracking it."
Brandford said most of these datasets are the main way of understanding deaths and how they happen - particularly in young people.
"We have tracks in our datasets of age of people, where they were in New Zealand when they died, what they died from, various ethnicities ... but we don't do that for people by virtue of their intellectual disability.
"So it's very hard to get a handle on why people with intellectual disabilities in New Zealand die so young, because we can't extract that information from those really important public health records."
The life expectancy figure was based on the most recent data from the Ministry of Health - which was published in 2011, which found people with intellectual disability had a life expectancy between 18 and 23 years less than the general population.
It also found that they were more likely to receive care or treatment for chronic health conditions, were more likely to use primary health services, were dispensed twice as many types of prescription drugs, and were four times as likely to have had avoidable hospital admissions.
Brandford said health agencies need to work together to improve the outlook. She said there should be a way to flag across a person's official health records that they have an intellectual disability.
"We do that in New Zealand for people with many issues - for example deaths by suicide, or deaths that occur post-surgery - and I think it's time we did it for people who have such poor health outcomes."
A recent investigation by the Chief Ombudsman, released in July identified significant gaps in the Ministry of Health's collection and use of information about the deaths of people with intellectual disabilities in full-time residential care.
The Ombudsman found that the Ministry of Health had not adequately dealt with deaths reported by disability service providers.
Brandford, who has more than 35 years' experience in the intellectual disability support sector, backed the Ombudsman's call for New Zealand to develop an independent national death review system.
Brandford's research was funded by the IHC Foundation.