Former Knox Grammar headmaster Ian Paterson has admitted he deliberately hindered a police investigation into claims of sexual abuse at the school.

During his appearance in the witness box today, Dr Paterson made a number of admissions, including that a number of boys under his care would not have been abused, had he taken action based on allegations of abuse that he was aware of.

In 1996 Inspector Beth Cullen, a police officer from the child protection enforcement agency, attended the school to investigate anonymous complaints of sexual abuse against five teachers.

Dr Paterson said he was aware of some allegations but said nothing at the time.

He suggested the detective should have questioned him more.

"I'm wondering why Inspector Cullen ... didn't question me more closely," he said.

The 81-year-old then admitted he deliberately hindered the investigation and protected the teachers.

One offender was teacher and convicted pedophile Adrian Nisbitt, who was rehired by Dr Paterson in 1989 despite earlier complaints.

"I said to him when I appointed him, 'I hope you are careful ... with your touching habits with boys'," Dr Paterson said.

Dr Paterson also denied that he sexually assaulted student Lucy Perry during a school play in 1989.

He said touching a bottom, in his view, did not amount to a crime.