RAAF fighter pilots
Lamb of God, if your mate was surfing near the RAAF Williamtown airbase he may well have seen FA 18 or Hawk aircraft flying fast and low. Near to RAAF Williamtown is Saltash Bombing range where the RAAF train. They also have designated low flying areas over land and over the sea. Additionally, when approaching to land into the west they overfly the beach in close proximity to the base. Don't worry about the boofheads with nothing better to do than throw crap at you, they were never sixteen years old and never made a mistake (aloud instead of allowed). By the way, it is not uncommon to have a low flying fast jet go by before you hear the sound.