AUSTRALIA is experiencing a rise in vitamin D deficiency, with the cost of testing increasing a hundredfold since the year 2000.

But some experts are concerned the testing - which cost Medicare more than $100 million in 2011 - might not be leading to better health outcomes for those found to be deficient.

Researchers from the University of Sydney and Westmead Breast Cancer Institute said the increase was not leading to tests and treatment for the potential consequences of vitamin D deficiency, which can cause brittle bones and has been linked to early death.

Kellie Bilinski and Steven Boyages also found evidence of over-testing, with nearly 15 per cent of people having four tests yearly, and more than 8 per cent of people having five.

One patient had 79 tests in one year, they wrote in The Medical Journal of Australia. Ms Bilinski, a senior clinical and research dietitian, said it was likely many were unnecessary.

If tests were limited to three per person yearly, based on the current costs, $20 million could be saved over four years.

''Our data show that more stringent guidelines should be developed for testing,'' she said. Further research was also necessary to establish whether the increased testing was leading to better health outcomes.

In a separate paper, published in the British Medical Journal, she said it was worrying that increased vitamin D testing had not led to increased bone density testing.

There are at present fewer than than 100 bone density tests for every 100,000 people, compared with more than 5000 vitamin D tests.

Rebecca Mason, a professor of endocrine physiology at the Bosch Institute for Biomedical Research at the University of Sydney, said one person having as many as 79 tests was ''ridiculous'', as there was generally no need for more than one unless treatment response was being measured.

''The increases in vitamin D testing have been largely due to the realisation that adequate levels of Vitamin D are important not just for bone and muscle but other health outcomes like mortality,'' she said.

Professor Mason said she was not overly concerned by recent research linking very high vitamin D supplementation and levels to death, as well as falls and fractures.

''It is something to keep an eye on but it's not something I would worry about,'' she said. ''In non-urbanised societies people would run around with vitamin D levels that were well above 100, and that would seem to be evolutionarily sensible.''

Current guidelines recommend an adequate level of vitamin D is more than 50 nanomoles per litre of blood.