A top Canadian security official says there was no ulterior motive for erasing tapes of audio surveillance of Air India bombing suspects, but looking back he wishes it hadn't happened.

James Warren, former chief of counter-terrorism for the Canadian Security Intelligence Service, said staffers were just following protocol.

"From the standpoint of someone who had to deal with the aftermath, I wish dearly they had not been destroyed,'' Warren said on Wednesday at the inquiry headed by former Supreme Court justice John Major.

"I think we all would have wished that they were saved, for whatever value they might have had in subsequent events.''

This week will be the final round of public hearings this year into how the 1985 terrorist bombing that killed 329 people, including 280 Canadians, was allowed to happen.

More than 150 tapes were erased by CSIS in the years that followed the bombing. In total, 54 recordings of phone calls made by Sikh extremist Talwinder Singh Parmar, who later became the prime suspect in the bombing, were kept.

The recordings were made several months before the flight went down on June 23, 1985, on its way from Canada to India.

Without the tapes, RCMP and Crown investigators were unable to review the full conversations for the criminal probes that took place later.

Though the tapes were erased before Warren took over, he said Wednesday that he had looked into why the tapes were erased and he never discovered "anything nefarious in the decision."

Instead, junior staffers were simply following the "default" policy where tapes are cleared after a certain period of time unless an order is given to save the material.

Former Crown attorney James Jardine said on Tuesday that CSIS failed to act on his requests for access to the information and that he never saw evidence any attempt was made to preserve it.

He said it was "inconceivable" that the tapes were erased.