New Zealand's Vietnam War veterans are almost twice as likely to suffer from a common form of adult leukaemia than the general public, a new study has found.

The Otago University study found veterans, who may have been exposed to toxic chemicals including Agent Orange when deployed in Phuoc Tuy province, have a 91 per cent higher incidence of chronic lymphatic leukaemia (CLL).

CLL is one of the most common forms of leukaemia found in adults, particularly older adults, and is rarely diagnosed in children.

The university research analysed medical records of 2783 of the 3400 New Zealand military personnel who served in Vietnam between 1962 and 1971. It found 0.5 per cent of them contracted the cancer, compared with an average of 0.26 per cent within the general public.

"The incidence of leukaemia is interesting," lead author Dr David McBride told AAP.

"The Australians and the New Zealanders appear to be the only group of veterans that have shown this excess."

Many Australians and New Zealanders fought in the same area during the Vietnam War and Dr McBride said chemicals known as Agent Orange or other pesticides used in certain areas could be the cause.

Scientists made the link between pesticides, including Agent Orange, and CLL during studies on farm workers exposed to the chemicals more than 10 years ago.

Despite this, scientists can't pinpoint exactly what causes the increased incidence of this cancer without information about individual exposure, five decades ago.

"It's a time and place associated with the disease, and therefore there must be something about it," Dr McBride said.

"But we're not sure exactly what it is."

The study also found lung cancers claimed the most lives in both New Zealand and Australian Vietnam veterans.

However, when it came to overall deaths, the mortality rate of the former New Zealand military personnel was 15 per cent lower than the general public.

Dr McBride says this is due to something called the "healthy soldier effect" which arises from the selection process of military members.

"This is related to the fact that this cohort would have been selected for its health and fitness," he said.