"The officers investigating the original complaints have made enquiries with past students and enquiries are ongoing," Detective Sergeant Neil Hilton of the Child Protection Team said.
"It is possible further charges may be laid."
A police summary of facts describes how Ng - who topped the country in school music exams in 2006 - taught the boys on a grand piano in a room of his parent's Epsom house.
But the offending took place in an adjoining office that was set up with a computer and camera on a tripod.
Within a couple of months of meeting the younger of the boys, Ng began smacking him on the bare bottom on a regular basis, "when he felt the victim had not been doing sufficient practice".
The boy told police it happened on 80 to 90 per cent of lessons.
Ng later shifted the blame to the student during police interviews.
"The defendant stated that what had happened had partially been the victim's fault, that if he had practiced more he would not have given him the opportunity to do those things."
Ng would sometimes put the boy over his knee and spank him or on other occasions he would force him to pose on all fours with his pants down, while he was distracted with a cell-phone game.
"Several times when the victim was being smacked, the defendant photographed and made video using the camera set up in the corner," a summary of facts said.
In late 2013, Ng picked up a teenage student from his east Auckland home for a pre-arranged "artistic photoshoot", outside the scope of their usual piano lessons.
He told the teen to bring his school uniform and asked him to remove more clothing as the shoot progressed.
"The poses that the defendant told the victim to make were sexual poses with an emphasis on displaying his bottom towards the camera," police said.
Ng also spanked the second victim but during questioning told police he believed the boy was 18 and consenting.
He explained his offending against the younger child by saying he was trying to make him practice more.
But after consideration, he told police "he developed a sexual for fetish smacking and was attracted to this act and feeling the victim's bottom".
Ng dodged questions outside court this afternoon but at his March appearance he told NZME he had always disliked children.
"I don't like the idea of touching other people, and kids especially not," he said.
"It's hard enough being a musician without this."
Ng will be sentenced in June.
