Azzira:
This forum addresses exclusively U.S. tax issues; the advice below does NOT include any Australian tax obligations.
That said, if you were once in the United States, but have separated from your husband and since returned to Australia, then you have NO OBLIGATION to file a U.S. tax return if you have NO U.S.-sourced income.
If you file jointly with your husband (an option since you are still married), then (and only then) would you be obligated to declare the rental income on the JOINT U.S. tax return.
If you file separately and spent part of the time in the U.S. and part of the time in Australia in 2011, you would file a dual-status return, in which case the rental income received while you were in the United States would be reportable on the resident portion of the dual-status.
Note that the dual-status return is NOT for amateurs; if you determine that a dual-status return is required, get professional help to file it.
If you need professional help filing your return, email me at
[email protected].