He downplayed the prospect of any combat, saying they would gather information about the insurgents, and any action against them would be taken by American special forces.
Mr Key said in order to act against the insurgents, "you need to build a case, it has to be within the mandate and the rules that operate there ... and that's what these logistics people do".
But Dr Buchanan, who runs a New Zealand-based strategic assessment consultancy, said he believed Mr Key's language didn't accurately reflect the work the SAS would do.
Logistics usually meant material supplies more than combat support. "It suggests the SAS guys will oversee the unloading of blankets at the PRT for the people doing the hunting. I doubt that's what they're there for."
Dr Buchanan suggested the presence of the four elite New Zealand soldiers may be intended as a show of force. "In war it's usually best to depersonalise any response to combat losses, but in a tribal society like Afghanistan all combat is personal and personalised revenge-taking is part of the game.
"Sending a small SAS team to assist the bigger army in the hunt for the bad guys sends a message to the Taleban and others that we might have been hurt and we may be leaving that theatre, but there is a hard price to pay for targeting us on the way out."
Dr Buchanan said it was significant the SAS troops were officers rather than enlisted men. He believed they may be used to bolster the leadership of the PRT in patrol situations.
"That would be advisable given the tempo of attacks on the PRT are expected to increase in the months ahead and most of the troops involved in the PRT's last deployment are relative newcomers to the theatre."
Labour defence spokesman Iain Lees-Galloway said it was "generally agreed that logistics is not the purpose of the SAS". However, by commenting on the deployment Mr Key was putting the lives of those soldiers and of our allies in danger, he said.
"It doesn't help to be alerting enemy forces to what our troops and American troops could be doing."
The 145-strong PRT force is due to be withdrawn in April, ending a 10-year deployment, while a New Zealand SAS force of about 70 was withdrawn earlier this year.
Mr Key said four SAS "logistics people" were sent to Afghanistan a week ago. Last month he indicated the deployment was likely after the deaths of PRT members Lance Corporal Jacinda Baker, Private Richard Harris, Corporal Luke Tamatea, and Lance Corporals Rory Malone and Pralli Durrer in two separate incidents.