Cult leader and self-proclaimed prophet William Kamm has been sentenced to an extra four years in jail for raping one of his former followers.

Kamm, 56, ordered the girl, then 14, to have sex with him, saying it was God's will because she was chosen by the Virgin Mary to help him repopulate the earth with his mystical seed.

The purported seer known to his followers as the "Little Pebble" is already serving a minimum three-and-a-half year term for an almost identical offence. Sentencing him to an additional four-year non-parole term on Friday, District Court Judge Peter Berman said Kamm took advantage of his position as leader of his cult near Nowra, on the NSW South Coast, to satisfy his own desires.

"Many people think that William Kamm is a prophet able to communicate directly with God and the Virgin Mary," Judge Berman said.

"This gives him enormous power over those who hold these deluded beliefs.

"He has taken advantage of these people by satisfying his desire to have sex with young girls."

Judge Berman said Kamm had used his religion in a deliberate plot to manipulate the girl's family into consenting to her chosen role.

Kamm, in a black suit and green shirt and tie, sat impassively as he was sentenced, with more than a dozen of his followers packing the small courtroom. When his previous sentence is taken into account, Kamm will serve at least seven and a half years in jail.

He will first become eligible for release in April 2013.