Defence chiefs say a big drop in the number of army personnel testing positive for drugs is the result of a new policy.

Seventeen soldiers tested positive in the first 11 months of last year, compared with eight navy personnel and one from the air force. The majority of positive results were for THC, the chemical found in cannabis, but three involved methamphetamine.

In 2009 there were 109 positive test results among army personnel compared with eight in the navy and none in the air force.

Defence Force Chief Lieutenant General Rhys Jones said comparing the changes year-on-year would be misleading. Testing had changed and positive results from medication were discounted.

He said any use by personnel was unacceptable.

About one in three personnel are tested annually.