Depends on if you applying for general or specialist registration, locums or permanent, area of need vs saturated areas.
Some good websites for general information are here:
www.amc.org.au
www.doctorconnect.gov.au
In general, here is how it would go from a US trained physician under a few scenarios:
1) After two years of residency, you can apply for registration in Oz under the competent authority pathway. Once approved, you can apply for a job - under this pathway, non-specialists and even some specialists, will usually undertake a rotating internship. IMGs are at the bottom of the list for obtaining an internship behind everyone else. Once you've got a job offer, you apply for limited registration. Upon completion the the year, you reapply through the AMC and, again once approved, can apply to the Medical Board of Australia for general registration. If you completed medical school and postgraduate training in the US and apply under the competent authority pathway, you do not need to sit the AMC exams (You are given an advanced standing certificate).
2) If you are US board certified, you can do (1) above, or you can apply for specialist recognition. Briefly, you need to contact the Australia College of your specialty (RACS, RACP, etc) and obtain a list of their requirements. You lodge an online application through the AMC for specialist recognition. They will have a ton of documents for you to send - medical diploma, intern certificate, residency certificate, board certification certificate, USMLE, Licensure verification, etc, etc. You also need to send the AMC all of the Australian's College (RACS, RACP, etc) requirements with you initial packet. The AMC will verify your credentials and then send on all the documents to the College. The College will assessment the documents and likely ask you to an interview. At that point, they will say how compatible your US training is compared with Australia and detail exactly what deficiencies need to be addressed. You can then apply for a limited license to the Medical Board of Australia. You will likely need to undergo a period of supervision (1-2 years) and may or may not be required to sit the College's Fellowship exam (equivalent to US Board Certification). Once you passed all those requirements, you can apply to the Medical Board of Australia for licensure as a specialist.
At this point, you are mandated to serve up to 10 years in a District Workforce Shortage Area/Area of Need in order to obtain reimbursement from Australian Medicare. You can apply for a waiver for other areas and these are determined on a case-by-case basis.
There is a considerable expense for all of this. AMC fees are a few hundred dollars for either the Competent Authority or Specialist Pathways. Each College sets their own fees for their assessment. This can range from just over $1k to $7k++. As a specialist with limited registration, you would also likely be required to pay an annual fee for the "supervision" until you are admitted as a Fellow to the College. This can vary widely (again) and is also dependent on whether the supervising physician/surgeon is local or remote. For the Royal Australasian College of Surgeons (RACS), this fee is between AU$5,865 to AU$16,745 - PER YEAR.
All in all, it is a very long, expensive and thorough process. The good thing is that you may not need to repeat your post-graduate training. Then again, some Colleges do require many IMGs to undertake some remedial training in Australia. If you were to fall into that category, you would need to find a hospital with a training program to fill your deficiencies (and you would be competing with Australian trained doctors for those positions).
Also note that there a few specialties which are notorious for giving very few IMGs advanced standing in this whole process. If you have not already started you US post-graduate training, I would suggest you call the Australian College of the specialty that you are interested in to see how favorably they view US Board Certification. Some have reciprocity, some will not really consider the training equivalent.
That being said, if the roles were reversed (and an Australian training physician/surgeon were trying to come to the US) - it would be an even bigger challenge since many US specialty boards will not allow IMGs to sit the exams if they did not completed their training in the US. Without US Board certification/eligibility it is nearly impossible to get on insurance plans or privileged at hospitals.
There is also the issue of the Visa through immigration, but that is another thread.