There are a number of Australian Embassies and High Commissions in South America. Below you will find the most commonly used Australian government representatives. The is also a listing for all the Australian Embassies in Asia if you are interested.
Note that it is best to call the High Commission or Embassy before you pay them a visit. This ensures that you have all the needed paperwork and also check that they will be open on that day. Many Australian Embassies tend to close on local (in country) public holidays as well as on Australian public holidays, so it is always best to check first. Most Embassies do not help with Australian Migration however you can ask for an immigration attorney in the country to assist you.