"The person whose duty it is to [handle the preparation of the head] is careful, when any of the skin of the face appears to wrinkle they smooth down and preserve it in shape," he wrote.
Art + Object rare book consultant Pam Plumbly said she'd had interest from New Zealand institutions as well as private collectors as far afield as the United States.
Repatriation expert Dr Cressida Fforde, who has worked for Aboriginal and New Zealand institutions, said it was not known whether Bennett traded in the heads himself.
Also up for auction are a series of autographed photos of Britain's war-time prime minister smoking cigars aboard the non-smoking BOAC Boeing Clipper RMA Berwick after a trip to Washington in January 1942 which helped chart the United States entry into the war.
Churchill and much of his war cabinet took off from Bermuda on January 16 for Plymouth because a sea journey across the Atlantic meant running the gauntlet of German u-boats. Still, a major concern was being intercepted by patrolling German Luftwaffe fighters over occupied France.
The flight made a hero of pilot Captain Kelly Rogers and was lauded by the Sunday Telegraph as the "most daring flight of the whole war".
Taken by the navigator on board, R.G. Buck, the pictures came from relatives of the airman. The find sent "shivers" up Art + Object's managing director Hamish Coney's spine.
A cartoon by S.D Moon lampooning the Germans, also signed by Churchill, could go for $5000 to $8000.