He said he had been told they would be playing an "active part in the hunt" for the insurgents.
"I've been told that the mission of these troops is not to gather intelligence but to help carry out the strikes or the raids on those insurgents that killed the PRT (Provincial Reconstruction Team) soldiers in August."
Mr Key said there had been no applications made to send more SAS troops to Afghanistan, and it would remain a "very small group" there providing "logistics and planning".
Mr Key said in September that four SAS "logistics people" were sent to Afghanistan, having previously indicated the deployment was likely after the deaths of Provincial Reconstruction Team members Lance Corporal Jacinda Baker, Private Richard Harris, Corporal Luke Tamatea, and Lance Corporals Rory Malone and Pralli Durrer in two separate incidents.
"If there is work to be done with insurgent groups then that will be undertaken by other SAS forces. Those SAS forces don't always have the time to do all of the planning work and our people know it very well so they can assist them to do, if you like, the behind the scenes work. But that's very different from picking up their guns and going on a mission."
New Zealand's SAS force of about 70 personnel was withdrawn earlier this year.