Former Tasmanian police commissioner Richard McCreadie says former Hutchins headmaster David Lawrence had admitted sexually abusing children and was due to be arrested when he left the school.

Mr McCreadie was a junior officer when he investigated the complaints of a former Hutchins student who said that Lawrence had abused him at the Hobart school.

Mr McCreadie told a royal commission that Lawrence had made a full confession and was due to be arrested when he suddenly left the school and went to the United Kingdom in 1970.

Music teacher Ronald "Mousy" Thomas had also admitted molesting students when he departed the school for South Africa around the same time, Mr McCreadie said.

Mr McCreadie said he did not believe the state government would have paid for the extradition of either men, and the case did not proceed further.

He said he did not discuss the matter with anyone else at Hutchins.

Mr McCreadie said he felt compelled to make a statement after seeing reports that the school believed that Lawrence had left because his secretary had opened a sexually explicit letter from a former student.