An air safety investigation has been launched to find out how a Cathay Pacific plane carrying 309 passengers ended up seconds away from a potential collision with another passenger jet as it flew over Australia. The Airbus A330 was on its way to Melbourne on December 22 when the cockpit crew realised that a Virgin Blue Boeing 737 was flying towards it on the same route and altitude over Australia's Northern Territory. The pilot of Cathay Pacific flight 135, almost full as expatriates headed home for Christmas, climbed 300 metres to 11,500 metres and veered to the right after alerting air traffic control. The crew of the Virgin Blue jet, flying from Melbourne to Darwin with 120 passengers, also spotted the danger and turned around to move out of the path of the A330. The two planes were 20 nautical miles apart at the time they changed course south of Darwin. With both flying at cruising speeds of 700km/h to 800km/h, they were only seconds from a potential collision when they took avoiding action. A spokesman for the Australian Transport Safety Bureau said yesterday it had classified the incident as serious and had launched an investigation, which is expected to take until April to complete. The investigation is expected to centre on the directions given by air traffic controllers and why they failed to realise the two planes were headed towards each other before the Cathay Pacific crew alerted them. The safety bureau spokesman confirmed that the cockpit crew of the Cathay Pacific plane, which left Hong Kong for the 11?-hour flight shortly before midnight on December 21, raised the alarm. 'When the crew of the A330 questioned the controller, the controller instructed the A330 crew to climb ... and cleared the aircraft to divert right of track,' the spokesman said. 'The crew of the 737 then advised the controller they were diverting 10 nautical miles right of track.' While the incident was not classified as a near-miss and cockpit warning alarms were not triggered, there had been 'a breakdown of separation standards' between the planes. A Cathay Pacific spokeswoman said: 'We can confirm that the incident took place and appropriate avoidance action was taken by the Cathay Pacific crew. The matter is now under investigation by the Australian authorities and we have already filed a report to the Hong Kong Civil Aviation Department.' Colin Lippiatt, spokesman for Virgin Blue, said: 'The two aircraft were 20 nautical miles apart when both aircraft altered course and the Cathay aircraft also changed altitude to ensure they remained a safe distance apart. The flight crew of both aircraft knew exactly where each aircraft was at all times. 'It is not at all uncommon for aircraft to pass each other during flight. This is why procedures are in place around the world to ensure adequate and safe separation distance between aircraft at all times. If anything, this simply demonstrates that those procedures work.'