A strange S-shaped formation which appeared on weather bureau radar off the West Australian coast was not caused by cloud, the bureau says.

The shape was spotted on the weather bureau's radar map on Wednesday about 30 kilometres west of Rottnest.

After checking the satellite, the bureau's Neil Bennett ruled out the phenomenon being caused by cloud.

"There's no cloud, there's nothing to produce a rain echo, ... which we do see a lot, but not this particular shape," he 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."

The ABC has asked the Department of Defence whether there are military operations off the coast which may interfere with radar activity.

The department is yet to comment.

The WA Weather Group has seized on the photo and retweeted it to their followers.

"Nice that our pet #RottNessMonster has avoided the shark baits. Her name is Susan & she likes to eat plankton," the group tweeted.

"And I, for one, welcome our new giant sea serpent overlords," Perth Sunrise Prints tweeted.