First, you are not an island, you are a continent. Second, the US Coast Guard does the same - with limited effectiveness (as I suspect the Aussie navy does as well).
Actually we're both - largest island in the world, smallest continent. The point I was making is that we have no land borders. You have to get in a boat to reach Australia, and you have to use an ocean-going vessel. To make port on our north coast you have to go to a small settlement, none of which are particularly welcoming of strangers (Darwin is our northern naval base, and so is not a clever destination). Anywhere else largely consists of jungle and, further south, desert, both filled with a refreshing combination of crocodiles, deadly snakes and highly venomous spiders. It takes training to survive in such an environment, which your average Islamic refugees doesn't have, and so many of those few who a)had a ship intact enough to make the distance and

manage to slip the radar net, either get caught in a settlement or die from the many, many dangers of our shores.
As a result most of our illegal immigrants are British, American, Irish and Canadian backpackers overstaying their tourist visas.
If you make it into Australia, you then have to deal with an immigration system which has a tendency to send our own citizens away just because they don't speak English.
It may be very hard to illegally immigrate to Australia in a blue-collar fashion, but that's as much a result of our environment as anything else. White collar tourists do it with ease.