This is not a difference in diagnosis. Tokelauans were examined for diabetes by the same group of physicians, using the same criteria. It's also not a difference in average age, sice the numbers are age-standardized. On Tokelau, diabetes prevalence doubled in a decade. Migrants to New Zealand in 1981 had roughly three times the prevalence of diabetes that Tokelauans did in 1971. I can only imagine the prevalence is even higher in 2008.
We don't know what the prevalence was in Tokelauans when their diet was completely traditional, but I would expect it to be low like other traditional Pacific island societies. I'm looking at a table right now of age-standardized diabetes prevalence on 11 different Pacific islands. There is quite a bit of variation, but the pattern is clear: the more modernized, the higher the diabetes rate. In several cases, the table has placed two values side-by-side: one value for rural inhabitants of an island, and another for urban inhabitants of the same island. In every case, the prevalence of diabetes is higher in the urban group. In some cases, the difference is as large as four-fold.
The lowest value goes to the New Caledonians of Touho, who are also considered the least modernized on the table (although even their diet is not completely traditional). Men have an age-standardized diabetes prevalence of 1.8%, women 1.4%. At the other extreme are the Micronesians of Nauru, affluent due to phosphate resources, who have a prevalence of 33.4% for men and 32.1% for women. They subsist mostly on imported food and are extremely obese.
The same patterns can be seen in Africa, the Arctic and probably everywhere that has adopted processed Western foods. White rice alone (compared with the combination of wheat flour and sugar) does not seem to have this effect.
The data in this post are from the book Migration and Health in a Small Society: the Case of Tokelau.
The Tokelau Island Migrant Study: Background and Overview
The Tokelau Island Migrant Study: Dental Health
The Tokelau Island Migrant Study: Cholesterol and Cardiovascular Health
The Tokelau Island Migrant Study: Weight Gain