They seem to be described the same way (a long sea serpent that eats stuff), and are commonly compared to each other.
Are there any reasons there are so many parallels?
Haha. great question on modern mythology! (I'd never even heard of the Hawkesbury River Monster.)
You might want to look into the ideas of Carl Jung regarding archetypes. He wrote a book Flying Saucers: A Modern Myth of Things Seen in the Sky which has been misrepresented by UFOlogists as a validation of beliefs regarding extra-terrestrials, but is really exploration of the common symbols that occur in visions people have when they experience such phenomena.
Remember that experience is subjective--we're in the realm of mythology, not science, but Jung is trying to find a rational basis for common experiences that are not strictly rational, and can't be explained in a scientific manner because there is still so much we don't understand about the mind.
The plesiosaur shape is one of the commonalities, but we don't really know how far back that goes. The most likely explanation is that the plesiosaur idea arose after paleontologists started reconstructing dinosaur skeletons. Before that, the frame of reference would have been dragons or other types of known, mythical creatures that fit into the aqueous environment.
If you look at UFO mythology analytically, you'll notice that there is a feedback loop between science fiction and UFOlogy. The initial conception of flying saucers was very similar to what one would see in early science fiction movies. The more movies, the more UFO sightings. The conception evolves in both areas, and there is a cross-influence.
Jung would say it all arises from the Collective Unconscious.
From a more cynical perspective, there are also economic factors. Nessie is a great tourist attraction, which boosts the local economy. The Hawkesbury River Monster may be less famous, but could also be a tourist attraction, and forging a closer association with it's more famous cousin has economic benefits.