For example, at the moment Downer and Stowe have the lionshare of the work...Their guys get paid the same we do obviously. My company has most of the work at the airport, but those jobs are expected to finish mid year. Obviously there's always more work to be won at the airport, but it's not a given that the contractor their currently will get the next job...
As my companies jobs finish, and if they don't win some more to keep the current amount of sparkies on the books, people will be made redundant (around 100 have already been made redundant in the 3.5 years I've been there) Some get re-hired, some are no good and don't come back, and others were just unlucky to be the one to go, and will more than likely get a job with whoever has most of the work at the time.
It's pretty rare that someone in my industry works for one company their whole career