info in
http://www.immi.gov.au/skilled/skilled-workers/sbs/

With this visa you can employ overseas workers for a period of between one day and four years.

With this visa those people you employ from overseas can:
•work in Australia for a period of between one day and four years
•bring any eligible secondary applicants with them to Australia – secondary applicants can work and study
•after entering Australia, have no limit on the number of times they travel in and out of Australia.