It is also vastly more urbanised than the US, most of Australia lives in a few areas that are pretty densely populated. We aren't all living in tiny communities in the middle of nowhere, Australia's population is hugely coastal.
It is an island is a silly response, we didn't shut borders before COVID got in so it isn't like us being an island is hugely impactful. Australia has done a really good job of handling community spread within Australia.
What proportion of the COVID cases around the world do you think are happening because of people from the outside a country visiting vs community spread to think the island thing is especially relevant?