Military police are investigating how unedited footage showing New Zealand soldiers in a fatal Afghan firefight was leaked on to the internet.

The expletive-laden footage, taken on a helmet-mounted camera and lasting for more than 10 minutes, shows the incident at Baghak in August last year where Lance Corporals Rory Malone and Pralli Durrer were killed.

It was released on to YouTube last week and differs vastly from a sanitised version released some months ago by the New Zealand Defence Force.

A defence force spokesman said the footage showed the intensity of the situation during the Baghak incident.

"The action that took place on 4 August was a very complex, chaotic and intense firefight. It is the most complex firefight NZ Defence Force personnel have been involved in since Vietnam.

"Our soldiers performed well under fire, our weapons systems worked effectively, international air support was able to be called in, and lives were saved as a result of the actions of our personnel.

"In spite of the extreme pressure that the soldiers were under, everyone who was involved in this contact displayed our core values of courage, comradeship, commitment and integrity."

The spokesman said the defence force had edited the footage to create a "comprehensible snapshot" of what happened and to render the footage suitable for public viewing.

"NZDF is aware the unedited footage is in circulation.There is no information in the film on Youtube which has not already been made public. The footage does not compromise or risk operational security."

The spokesman said Military Police were investigating how the footage was posted, including whether a serving defence force member was involved.