Sydney, Aug 14 : The head of Australia's transport safety investigator said on Friday that the agency is trying to understand why a safety incident involving two planes at Sydney Airport on Sunday was reported late.

The Australian Transport Safety Bureau (ATSB) said on Thursday night that it has commenced an investigation into a runway incursion incident on August 9 involving Qantas flight QF728 to Ayers Rock Airport in central Australia and Virgin Australia flight VA555 to Perth.

A statement from the ATSB said that air traffic control cleared the Virgin Australia flight for a full-length runway take-off from Sydney around 10 am local time on Sunday. As the crew applied thrust, they observed the Qantas flight begin crossing the runway ahead of them, Xinhua News Agency reported.

The Virgin Australia flight subsequently came to a stop before being re-cleared to commence take-off.

ATSB Chief Commissioner Angus Mitchell told Australian Broadcasting Corporation radio on Friday that he was not notified about the incident until Thursday.

"The two aircraft didn't come within, really, probably around a thousand meters of each other. But, notwithstanding that, it shouldn't have occurred," he said.

"We're still trying to understand exactly what the delay in the reporting to us was."

The ATSB said on Thursday that the Qantas crew involved in the incident on Sunday reported having received clearance to cross the runway.

It said that investigators would interview involved parties and review data.

The incident was the third runway incident at Sydney Airport in two weeks.

Earlier on Sunday, a Jetstar Australia flight was forced to brake suddenly to avoid colliding with a Qatar Airways jet that was being towed, while on July 27, a QantasLink flight and a Qantas plane were involved in a near-collision.

A crew member was injured in a near-collision between a Jetstar Airways flight and a Qatar Airways plane on a runway at Sydney Airport on Sunday morning.

Jetstar flight JQ402 to the Gold Coast was taxiing on the runway prior to taking off around 7 am Australian Eastern Standard Time on Sunday, when the pilot was forced to brake suddenly to avoid colliding with an empty Qatar Airways Boeing 777 that was being towed.

A Jetstar cabin crew worker was injured in the incident and treated by ambulance paramedics. No passengers were injured.

A spokesperson for Jetstar, which is a wholly-owned subsidiary of Qantas, said that the airline has commenced an investigation into the incident.

"Our aircraft was following instructions by Air Traffic Control when the pilots were forced to brake firmly after another aircraft came in close proximity," they said.

/IANS