You must get a visa to do your internship in Australia.
The easiest visa to get is the Working Holiday Visa (WHV). This visa limits the duration of the internship to six months in the same company and to one year in all on the Australian territory. If you cannot stay more than six months in the same company, you can do two 6 month-internships in two different companies.
If you wish to stay more than 6 months in a company, you will need to ask for an Occupational Trainee Visa Program. This visa can be asked while you are in Australia.
The Occupational Trainee Visa is more difficult to get than the Working Holiday Visa because the company needs to write a full internship program and get this program validated by the Adelaide Occupational Trainee Processing Centre to become a sponsor. This will cost $1000 to the company. You will aslo need to ask for this visa at the same time as the company.
Unlike the Working Holiday Visa, the Occupational Trainee Visa allows you to stay two years in the same company. You don't have the right to leave the company for another during this 2 years-time.
There are no quotas for the Working Holiday Visa Subclass 417. Every people aged between 18 and 30 can apply.