A territorial soldier who went to Afghanistan as a "bean counter" ended up leading security patrols and was hauled before a Court of Inquiry when he lost a high-tech assault rifle.

The Sunday Star-Times has learned that reserve force Lieutenant Tim Easton - who now works as an executive assistant to Labour's Clayton Cosgrove - lost his IW Steyr rifle while on patrol in Bamiyan province last year.

It is understood Easton had two weapons with him and realised one was missing when he returned to base. Troops returned to the scene and tried to retrieve the lost Steyr, to no avail.

A source said it was thought that locals had recovered the weapon, but despite negotiations, refused to hand it over. It is feared the gun, with a 30-shot magazine, might have ended up with insurgents and been used against coalition forces.

Losing a weapon is highly embarrassing for a soldier and Easton told the Star-Times he did "not feel comfortable" talking about it.

The Defence Force confirmed in a statement that the gun was lost "and despite intensive searches, was not recovered".

A Court of Inquiry was conducted and "appropriate" disciplinary action taken.

The statement said a range of weapons were "easily obtainable" in Afghanistan but the ammunition required for the Steyr was not readily available to locals.

"There is no evidence that the lost weapon was received or used by insurgents."

Easton was employed at Defence headquarters in Wellington as a business analyst when he was posted to Afghanistan to work as a finance officer at the provincial reconstruction team's headquarters, a civilian role.

Because of his background as a Territorial officer he was allowed to do security work. The Territorials, colloquially known as "Weekend Warriors", are New Zealand's reserve force and there are about 1850 of them.

The Government is restructuring the reserves so they can take on a greater load to save costs.

Easton told the Star-Times he had led "a couple" of patrols in Afghanistan.

The source questioned why a reservist was allowed to lead patrols on such a dangerous and complex deployment, as the gap in training between a reserve force officer and regular force officer was "huge".

Easton's posting "raised eyebrows" at the time. "He was basically supposed to have been a bean counter over there," the source said.

Afghanistan was New Zealand's deadliest conflict since Vietnam, with 10 soldiers killed. A few months after the gun was lost, New Zealand troops were ambushed at the "Battle of Baghak", with two killed and six injured.

Former army chief Lou Gardiner, who led a review in 2010 that recommended a greater use of reservists, said it was "very unusual" for someone posted to an administrative role to go on patrols and he would be surprised if that happened.

Easton told the Star-Times: "The way the army's moving now is to be inter-changeable between reserve forces and [regular] forces. The training that I had is the same, the length and service I guess was a bit different. But as a Lieutenant going over there I was . . . qualified to do the roles that I was doing."

The source disputed that, pointing out that a Territorial Force commissioning course ran for seven weeks compared to 11 months in the regular force.

Gardiner said: "Reservists are trained to a certain level, it would take them longer to get up to what a regular force guy is. It depends on whether he had done previous training and some full-time service and got up to speed. He wouldn't have got up to speed in a 30-day pre-deployment situation."

A Defence Force spokesperson confirmed Easton participated in patrols "on a few occasions", including commanding one escort in a low threat area.

He was a qualified infantry platoon commander, had completed pre-deployment training and a "robust" command training module, the spokesperson said. As a Territorial he had done the officer commissioning course and the infantry platoon commander's course.

Easton left his civilian job at defence soon after arriving back in New Zealand, but said that was not part of the disciplinary action.

Cosgrove said he did not know about the lost weapon and the intricacies of an employee's past jobs were not his business. The last New Zealand soldier from the PRT left Bamiyan in April, although a small number of soldiers remain in Kabul working with the International Security Assistance Force.