Australian authorities are investigating two more aircraft taxiing incidents at Sydney Airport, adding to a string of near misses and other mistakes on the tarmac that have raised concerns about safety.

Early on Aug 25, a Jetstar plane that was leaving its gate almost crossed paths with a second aircraft that was taxiing behind it, the Australian Transport Safety Bureau said Wednesday. In an earlier incident on Aug 16, two recently landed Qantas Airways Ltd and Virgin Australia Holdings Ltd jets had converging routes, before the Qantas crew spotted the other plane and stopped.

The Virgin jet had been told by air traffic control to give way to the Qantas aircraft, but the instruction wasn’t read back by the crew and air traffic control failed to spot the mistake, the safety bureau said.

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The latest events compound concerns about a potential collision and highlight a persistent shortage of air traffic controllers at Sydney, Australia’s busiest airport. The system is showing signs of nearing a breaking point after at least six incidents in the past month.