Paddy is correct although the fact that cabin crew are on the move all the time might help reduce the risk. I do not know the figures however what I would suggest is to wear support tights if you are a girl and appropriate socks if you're a boy.
On my very first flight as crew I wore the normal 20den stockings as per crew uniform standards. Guess what? it was a 11hour flight and I ended up with terrible pains in my legs and a swollen vein behing my left knee. I spent the first 2 days of my layover trying to recover!!
I thought I would develop if anything else some varicouse vein problems but I started wearing proper support stockings (you can buy them in pharmacies in Italy, quite expensive but so worth the money and last a long time) and 5 years later that vein hasn't got any worse. Even carrying a 10lb baby didn't do anything to it as I kept wearing that stuff during the 9 months.
I have seen socks for boys which use a similar concept and apparently can help you.
I do not know if Qantas have any particular rules about whether you can fly or not, maybe someone who works for them can help?
All the best
FBW
