I like this definition. Not sure that the "non-intersecting" condition is necessary. I was thinking about a different one, given by the condition that the path never goes farther than d meters from the sea (while encircling the part of the land that is farther than d meters).
A minimal-length, non-intersecting loop for which every point on the coast is within d metres of at least one point on the loop.
The only parameter is d, which I think most people would happily set to 5 or 10 km for a country the size of Australia.
It may not be unique, but I think anything obeying this would count.