The Green Card is the equivalent of the Australian Permanent Residency status. A lot of individuals seek the Green Card because it allows them the ability to move freely between jobs without having to return to Australia to seek a new Visa. Moving between jobs while having the "visa approval" restriction can be very daunting for individuals. Some larger companies (like Facebook, Google, etc) are accustomed to handling this problem, but most smaller companies find this to be a risk when interviewing a candidate.
A green card is typically obtained from:
- the Diversity Visa Lottery (the Greencard lottery), which occurs once per year.
- employer sponsored Green Card. This requires an employer who is willing to jump through all of the hoops and pay the legal fees to get your employer sponsored Green Card. Often an employer's incentive is to keep you on a Visa to limit your abiltiy to move freely between jobs.
Getting a green card involves renouncing your Australian citizenship (even though you can hold dual citizenship!) and pledging unconditional allegiance to the USA. Although their is no explicit requirement to hand over your Australian Passport and the Australian government will always guarantee you that you can have an Australian passport, regardless of what the US demands.
There are significant tax consequences to consider in Australia and in the USA and "handing back" your Green Card can be a very expensive thing to do. Below are two articles that explain why.