RAAF had a boom nozzle seperation about 2 years ago, not a complete boom separation. The nozzle broke away at the frangible part, as designed, during a receiver training sortie.
There have been 2 MRTT complete boom separations: the first was on a RAAF aircraft flown by an Airbus crew during a training sortie with a Portuguese F16 and the second on a UAE aircraft flown by an Airbus crew during flight test.