King's headmaster Tim Hawkes has apologised to his school community and admitted he was stupidly insensitive to say he had "no regrets" about his actions almost 30 years ago at Knox Grammar, where he failed to report the sexual abuse of a young boarder.

"There are moments in your life that you wish you could go back and change,"  Dr Hawkes wrote in a letter to King's School parents on Tuesday. "There are moments when you cannot believe your stupidity. There are moments when you want to run away.

"I was met with such a moment last week when being pounced on by the media when leaving the Royal Commission looking into the Knox case. How on earth can you disentangle the appalling impression given that, 'I had no regrets' in relation to a Knox matter some 27 years ago.

"Regrets! I have thousands."

Dr Hawkes had given evidence to the royal commission into child abuse that, while he was a housemaster at Knox, he was unaware of the mandatory reporting laws for child sexual abuse introduced in 1988, soon before a 14-year-old boy was groped by a man who had hid under his bed, wearing a balaclava and an old Knox Grammar tracksuit.

Nor did Dr Hawkes confront two teachers who had been possible suspects, Chris Fotis and Damian Vance, but he believed he had done his duty by reporting the attack to then Knox headmaster Ian Paterson. Nobody reported it to authorities.

In his letter to his school community, Dr Hawkes said: "My heart breaks for the boys whose trust in some Knox staff was betrayed. I, along with much of Australia, have been appalled at the revelations coming out of the Royal Commission. Regrets? None of it should ever have happened.

"I can make no excuses for my comment. I was tired after three hours in the witness box. I was alarmed - even frightened by being ambushed by the media scrum, and greatly angered by the hostility of their questions.

"I should have said absolutely nothing. What I tried to say was that I had no sense of not having done the right thing by reporting the matter to the headmaster at the time. However - it did not come out that way, and people, quite rightly, were appalled at the apparent heartlessness of my comment.

"The media have offered to re-interview me. That is kind, but it will come across as a revisionist attempt by me to make me look good - and that thought appals me.

"Regrets? Absolutely! Regrets for an insensitive remark – yes  – but even more so, for the things that happened at Knox. There needs to be justice given, care provided and lessons learnt – not least, by me."

Mr Fotis remains at large after a warrant for his arrest was issued last Wednesday, following his failure to appear before the royal commission the previous day.