Two Qantas planes involved in a near-miss over Adelaide on Friday were just 25 seconds from a disastrous mid-air collision, according to the Australian International Pilots Association.

QF581 was flying from Sydney to Perth at 38,000 feet and QF576 from Perth to Sydney was flying at 39,000 feet when the lower-flying craft received permission from an air traffic controller to ascend to 40,000 feet.

At that point, the collision warning computer on the other aircraft went off, advising it to climb to avoid a collision.

"The computer warning system is time-based," said Richard Woodward, vice-president of the AIPA. "But typically it's about 25 seconds head-on. It'll go "climb now" or "descend now", and that's a resolution advisory. They got one of those, so I assume they were within 25 seconds of each other.

Mr Woodward said the two aircraft would have been flying towards each other at about 32 kilometres a minute, or one kilometre every two seconds. When the warning went off, "they would have been within 10 or 15 kilometres of each other", he said.