Not Australian so don't know much about the E-3, but it seems Canadians have a similar visa (TN-1) - indefinitely renewable also.

The trick is that it's a non-resident-intent visa, which is to say, while it's indefinitely renewable on paper, at some point USCIS is going to point out that you've been here for an awfully long time and you risk the renewal not going through.

Not sure if the same applies to E-3.