If taxes are withheld in Australia on dividends declared by Ausrtralian companies, then are the same dividends tax free in the U.S,?

No, as a "US taxpayer" (US citizen anywhere in the world, or a non-citizen who lived in the US more than half the year) you are required to report and pay taxes on all your world-wide income.

However, if you also pay foreign taxes on the same income, you can report that in Turbotax for a tax deduction or credit.