US License
The task of converting an Australian licence to a US license is easy enough but finding the information is not. I have contacted several FSDO's and have obtained conflicting information but as far as I am aware the following is pretty close.
These days the FAA will only issue a PPL, regardless of what foreign license you hold(min ICAO PPL).
Fill in the verifcation form noting which FSDO you will attend. They will then send you a letter allowing you to visit said FSDO office in the next 6 months.
When you visit the said FSDO office they will issue you a PPL based on your Australian license (SEL & MEL if held) and it will only be valid when carried with your Australian (valid) license. You can use the US PPL subject to completing a BFR(in the US). If you intend to rent an airplane in the US, you want to seriously look at taking out renter's insurance!!!
If you want to add an Instrument rating, at the time of the visit point out that you have an Australian Instrument rating and that you have completed the IFP, Instrument Foregin Pilot written exam within the previous 24 months and they will include the rating.
If you wish to then get a commercial or ATP, it is a "simple" matter of sitting the appropriate written test for the license and completing the flight test.(Assuuming you meet the minimum hours requirement). Someone like ALLATPS will do this for around US$1195(ATP). To add an Instrument Rating to the Commercial or ATP I belive you will have to sit the IRA, Instrument Rating written.
I presume that if you do the IRA initally instead of the IFP, that this would still get you the Instrument rating on the PPL.(IRA covers a greater subject matter)
Hope this helps...........