They are an agency that Virgin Blue use to recruit ground staff. Many of the ground staff employed are not actually employed by Virgin Blue, they are employed by Westaff. Melbourne ground crew use them also. I think after you prove to be a hard worker/team player, you then get offered a full time position with Virgin.
Its kind of like the Qantas/MAM thing, you wear the uniform but are not actually employed by the airline.
At least that is my understanding of it.