Despite resembling plasticky four-leaf clovers, these are actually aquatic ferns. The strong resemblance, though, is reflected in some of its common names: water clover or floating water clover. It is also known as the banded nardoo.
This is a plant of ponds and riverbanks throughout most of Australia, including Tasmania. The flat, broad leaves are representative of one of the two typical leaf morphologies for aquatic plants. Large, flat leaves aid in both flotation and optimal capture of sunlight, and are often found at the water surface. The second typical aquatic leaf form of submerged, fine, narrow leaves is not observed in Marsilea (for an example of such, see Elodea)
Photographs of the plant in habitat can be seen on the New South Wales Flora Online: Marsilea mutica.