A highly-decorated former NZSAS commander has opened up about the Kiwi unit's military operations in a new memoir.

Serviceman J details Jamie Pennell's recruitment story and training journey - and lifts the lid on the NZSAS's Afghanistan operations, including the deadly Kabul hotel siege.

Pennell says the selection process to join the unit was a difficult one - mentally, physically and spiritually. 

"It's very brutal - and that's by design. Operations are quite difficult, so we have to replicate that in our selection process."

LISTEN ABOVE