You will be taxed as a non resident. If you have an Australian Tax File Number you should give it to them and they will withhold tax here at the non resident rates. If you do not give them a tax file number they will have to hold back 50%.
Because you are taxed here, in the US you will have that tax credited to you under the Tax Treaty the tow countries have.
Here is the link with the key points
But you should give them the tax file number for australia if you have one.
If you are not an employee but a contractor they should not withhold tax but you issue them with a tax invoice and they pay you.
Your work is done overseas and therefore no GST is collectable by you.
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