I would assume that Australians living near the coast do so for much the same reason that anyone would like to live near the water. Being a native and long-time Floridian and currently living 80 miles from the nearest beach, I find the distance too far for my own desires. The lure of the water is a great one. It provides recreation--particularly swimming, boating and water sports--as well a beach for other similar pleasures. The coastal waters create cool winds in the summer months. For most people, the vista is intensely pleasurable. Living near the coast creates easy access to seafood, whether purchased in a store or caught by the individual.
As for Australia, the island is a vast and arid one, with deserts making up a great deal of the land mass. All of the nation's major cities are found on the various coasts, and these cities and their inhabitants are there for many of the same reasons as listed above. Inland towns and communities are remote, and most of the island's population live along the coast. Historically, most early towns and cities originated on the coast or on rivers due to the easy access by ship. Even in the era of the plane and automobile, shipping is an important form of economic transportation, another major reason for Australia's large coastal populations.