I agree fatty, but I know some successful applicants were hearing their referees had been contacted 7 days after the assessment day. That's a pretty good indication that you're in, that they've decided they want you.
I know a lot of employers hold off turning down the unsuccessful applicants, in case someone turns down an offer of employment and the next person in line can get an offer. It's not the same with this job, you're either suitable or you're not. If they know enough to be contacting referees after 7 days for the successful applicants, they know enough to put the unsuccessful ones out of their misery. It's not like they need to ring them up and say "sorry, you weren't good enough". I wouldn't wish that job on any recruiter, that's why the vast majority of employers will tell you via email or snail mail.
The only reason I can think of for them making the unsuccessful applicants wait, is that after 4 weeks of waiting they've pretty much resigned themselves to the fact they aren't in. If you get an email a week after your assessment, when you're still on a high thinking you did well, then you're gonna fall a lot further, when they tell you no.
Personally, I'd rather be told early, so I can concentrate on other things. A job with Airservices is not the holy grail, and it was never portrayed that way during the whole selection process.