A blurt is an Australian fart. Generally, a good blurt has a low pitch, and is of medium duration. The type where the gas escapes at just the right ratio of pressure to volume to cause the arsecheeks to get up a sort of rhythmic clapping motion, and the gas exits more or less as a series of pulses, to the great amusement of your mates. A good blurt when seated on an empty 44-gallon drum produces a sound like an orchestral drum-roll.
Bluey and Curley, two brickies labourers, are sitting on a timber plank, eating their lunch. Curley rips out a classic blurt, and the vibration along the timber causes Bluey's Vegemite sandwiches to rattle off the end of the plank.