There are 2 results of your search for rock lobster1.
rock lobster1
noun a marine crayfish.
Contributor's comments: Rock Lobster is a name forced onto us West Aussies by the marketing gurus. The local name is "Crayfish" or "Cray" which of course was caught with a "cray pot" or shot with a gidgie.
rock lobster2
noun A rock lobster is a colloquial expression for an Australian $20 note (because of its red colour)
Contributor's comments: The $20 note was also called a "redback" and the fifty a "Yellowbelly".