Neil Bennett, an expert with the bureau, said the shape appeared on the bureau's radar map Wednesday west of Rottnest and officials compared it to satellite pictures and determined the mystery shape was not a cloud, the Australian Broadcasting Corp. reported Wednesday.
"There's no cloud, there's nothing to produce a rain echo ... which we do see a lot, but not this particular shape," Bennet said. "They don't take on S shapes and things like that. The radar that we use are there for the detection of precipitation, it's basically just a beam going out and hitting the rain droplets or ice particles from hail."
"Sometimes the beam itself rather than going straight it gets bent back to Earth and you start to pick up reflections from the ocean, rather than rain droplets," he said.
The Department of Defense did not immediately respond to a question about whether any military operations could have interfered with the radar, ABC said.
The West Australia Weather Group joked about the picture on Twitter.
"Nice that our pet #RottNessMonster has avoided the shark baits. Her name is Susan & she likes to eat plankton," the group tweeted.