Twelve sexual assault and harassment complaints - including seven for alleged "sexual violation/rape" - have been reported to New Zealand's military police in less than a year.

Figures released under the Official Information Act reveal a dozen investigations into serving military personnel were launched between July 1, 2015 and May 31 this year.

According to the documents, 10 investigations are ongoing, one resulted in a guilty verdict by military summary trial and the result of the other completed case is not known by military police.

Almost all the alleged incidents were against women, with one complaint from a male army staffer.

Seven alleged offences are classified as "sexual violation/rape" - detailing a rise in the most serious sexual allegations levelled over the past two years.

Annual statistics reveal eight complaints reported to military police in 2014, three for "sexual violation/rape".

The following year the total number of complaints almost doubled to 15 and the number of "sexual violation/rape" cases almost tripled to eight.

The figures have energed after a string of drug-related incidents among the defence force.

Last month three Royal New Zealand Navy staff were stood down over the alleged use of illegal substances while on deployment in Cairns.

"Three have been sent back to New Zealand and arrangements are being made to send home the fourth member at the earliest possible opportunity. No further comment will be made until the investigation is completed," a spokesman said.

The Defence Force also recently stood down five personnel while probing an incident involving an unknown substance while on operation in Fiji.

In December five soldiers were fired after taking a psychoactive substance, commonly known as NBOME.

Labour's defence spokesman, Phil Goff, said the recent drug incidents and rise in sexual complaints would be of serious concern to NZDF chiefs and must be addressed.

"The NZDF will be worried at the trend shown in sexual assault and harassment cases in the last half dozen years.

"The number of alleged rapes, in particular, that represents a four-fold increase over the last five years," he told the Weekend Herald. "If you put that on top of the eight drug cases involving military personnel overseas, there is clearly a problem."

Goff said the NZDF needed to get to the bottom of whether it was a "few bad apples" or a systemic problem.

In a statement to the Herald, Chief of Defence Force Lieutenant General Tim Keating said recent behavioural issues were unacceptable.

"I have made it very clear on several occasions, through both internal and external communications channels, that even one complaint of sexual assault and/or harassment is not just a matter of concern, it is unacceptable to me and the NZDF's senior leaders, both military and civilian."

In March, NZDF launched Operation Respect, an action plan aimed at sexual assault prevention. Keating said he believed it would produce results.

"I am fully committed to its success. It is both a moral and operational imperative, and I believe that the actions we have already taken and those to come will raise our ability to understand the issues, support victims, respond to incidents and of course to prevent further occurrences.

"To achieve these outcomes is vital to the NZDF's culture of dignity and respect for all its members," he said.