J.D., Haddon.

Your yellow flower belongs to common wedge-pea, a small plant in the pea family. Its bright, clear yellow flowers are larger than those of most other native peas, and also slightly later than most, appearing from late spring and into summer.

Plants from south of Ballarat are usually shorter than 30cm high. But north of the Divide they grow taller, sometimes to 60 or more centimetres.

Your sighting of just one flower was probably the result of this plant flowering earlier than most. The plant is not uncommon locally, but because it is small and slender it is often not noticed until its bright flowers appear.