The collection will be put up for auction by Sotherby's on July 10.
According to auctioneers, the collections is expected to be auctioned for between 500,000 and 700,000 pounds.
In one of the letters, Gandhi writes on Kallenbach on 25 March 1945: "...He used to say to me often that when I was deserted by the whole world, I would find him to be a true friend going with me, if need be, to the ends of the earth in search of Truth..."
The Catalogue Note on the collection says that it "is richly informative of the important (and occasionally misunderstood) friendship between the two men, and is a key biographical source for Gandhi".
Kallenbach (1871-1945) met Gandhi in 1904 in South Africa, and the two since then remained in touch even after Gandhi returned to India.
In 1910, Kallenbach purchased a 1,100 acre farm at Lawley, 20 miles from Johannesburg, which he gave to Gandhi.
The two were closely involved in operating the farm.
The archive for auction not only includes documents relating to the purchase but also reveals the two men acquiring and tending fruit trees, ensuring water supply, and arguing with neighbours over grazing rights, Sotheby's said.
The archive is described as including "poignant letters" by the deeply troubled Harilal, Gandhi's first son, and reveals Kallenbach's deep friendship in particular with Gandhi's second son Manilal, who remained living on Phoenix Settlement in South Africa, and his third son Ramdas.
"These letters, together with those by other family members, Mahadev Desai, and other of Gandhi's close associates in India, provide a detailed portrait of Gandhi's personal life in India," the Catalogue Note says.
"This extraordinarily rich archive stands... As a testament to a hugely significant figure in the life of Gandhi and a key member of his inner circle," it adds.
London: A large collection of letters, documents and photographs related to Mahatma Gandhi's time in South Africa, especially related to his controversial relationship with architect Hermann Kallenbach, is expected to fetch between 500,000 and 700,000 pounds at an auction here.
First Published: Thursday, June 14, 2012, 15:47