Greer told the board that naked young girls did not sexually interest him, despite police finding 17 photographs of six different females in "various stages of undress exposing their genitals or breasts" on his digital camera.
He denied selling or giving the girls drugs in return for exposing their bodies to him.
"We do not believe his denial," said the Parole Board.
"His criminal propensity to deceive is illustrated in his extensive (over 100) fraud/deception related crimes."
Psychologists recommend that Greer, who has a "depressing"criminal history, with 161 convictions since 1983, undergo more intensive one-to-one counselling to gain an insight into his offending and develop a strong safety plan.
"Very much more is required on the part of Mr Greer to address his offending behaviour, distorted beliefs and reduce his risk to something below undue," the Parole Board said in a decision released to the Herald today.
"In our judgment, it is of such a level that he does not meet the statutory criteria for release on parole.
"What treatment is afforded to him is a matter for Corrections but it must be undertaken to reduce his risk."
Greer, of Bryndwr in Christchurch, pleaded guilty to charges at Christchurch District Court that between January 1 and May 24, 2013 Greer "did enter into a dealing" with two girls under the age of 18 for the purpose of sexual exploitation.
He also admitted supplying cannabis, as well as three charges of possessing cannabis and the Class B controlled drugs morphine, and Ritalin. He was sentenced at the High Court.
Police watched the girls enter the van, which had curtains guarding the illicit deals in the back, before they pounced on Greer.
They found two teenagers who admitted allowing the photographs to be taken in exchange for drugs and cigarettes.
Officers also found $14,000 in cash stashed inside the van, as well as thousands of dollars worth of cannabis, morphine and Ritalin.
Text data recovered form Greer's cellphone established he had sold cannabis to 20 "school-age students" over a five-month period.
In jailing Beria, Justice John Fogarty said: "The long term effects on the community of supplying cannabis ... to students is very serious."