An arrest warrant has been issued for a former Knox Grammar School teacher who failed to appear at the royal commission into child sexual abuse.

Christopher Fotis was called to give evidence at the Sydney hearing on Tuesday, but did not show up.

Counsel assisting the commission David Lloyd said New South Wales Police was investigating, and Mr Fotis may be brought in to give evidence while in custody.

"On Tuesday morning we called Mr Christopher Fotis up and there was no appearance," Mr Lloyd told the hearing.

"In the wake of Mr Fotis's failure to answer the summons issued on him by the royal commission, the royal commission has issued a warrant for his arrest.

"The warrant remains outstanding at present and the commission is currently receiving updates from the New South Wales Police in relation to the warrant."

The hearings are looking at the way the prestigious boys school, in Sydney's north, handled complaints of inappropriate conduct by teachers over more than 30 years.

Earlier this week, the inquiry was told five Knox teachers had been convicted of child sex offences, and there was evidence three other teachers were involved in inappropriate conduct but had not faced criminal proceedings.