Screening rates for Type-2 diabetes are so low that as many as a half-million Australians may have the disease, but they don't know it.

Only five percent of Australians over the age of 40 have had a check for Type-2 diabetes in the last two years, according to Diabetes Australia CEO Greg Johnson.

"The high figures are really startling and alarming," he said in an interview with the ABC.  "The critical thing here is many people actually have type two diabetes for up to seven years before it's diagnosed and during that time it could be silently doing damage to your body.  If we detect it early and if we treat it we can actually prevent most or all of the complications."

Diabetes leads to terrible complications such as preventable blindness, limb amputation, and end-stage kidney disease.  And it's largely avoidable because diagnosis is a simple matter.

"I really recommend you add a diabetes test your next visit to your GP," said Diabetes NSW and ACT chief executive Sturt Eastwood.  "And if you are over 50 or you have anyone in your family with diabetes I would recommend you start that process today."