Australian's vitamin D levels drop to their lowest at the start of spring, suggesting testing regimes need to be changed, experts say.

A University of Sydney study that examined 24,000 NSW patient samples between July 2008 and July 2010, found vitamin D deficiency ranged from 33 per cent in summer to 58 per cent in spring.

Vitamin D levels peaked in summer before declining gradually in late summer and falling sharply in the winter months, the study found.

But levels dropped to their lowest ebb in spring, not winter as previously reported.

Professor Steven Boyages from Sydney Medical School said current testing guidelines did not take seasonal variation into account and might need to be reconsidered.

"Ideally testing would occur in spring when vitamin D levels reach their lowest concentration," he said.

"If an individual is found to be deficient a subsequent test three months afterwards would see if they have been able to replenish their vitamin D."