Some judgments — most notably Michael Kirby’s decisions in the High Court — refer to United States judgments in their reasons.
I confess I have no familiarity with the US legal system other than a vague understanding they have a Federal system similar to ours. (Here’s an interesting aside: did you know that aside from Australia, only the USA and a handful of other countries have States? Every other country in the world subdivides itself into provinces or something similar! This is often a source of confusion for Australians when we deal with international law — such as human rights law — where states are synonymous with nation-states.)
My former university has a very straightforward yet informative site describing the US legal system and how to conduct basic US research. Although we don’t have any access to these resources, it means we can at least understand the application of US law when we encounter it.
To make sense of the citations, the New York University School of Law has a Guide to Foreign & International Legal Citation available for free download.