Q: How much will I get?
A: You will get the difference in pay over the number of pay periods the retroactive period covers. We were due for the last pay rise in April 2004, if I am not mistaken.
Therefore, if you are a reservist, you will get the difference between your 2004 pay, and the new pay rate, times the number of half-days and full-days you worked since April 2004 (or whenever they made it retroactive to).
If you are regular force, the pay rates are monthly, and the same idea applies. The number of months since April 2004 until now, times the per-month raise you got.
That should give you a pretty good gross estimate. They will, of course, make all the usual deductions (the big one being tax, and if you qualify for it, superannuation and other benefits).
The delay is likely due to the sheer size of the army and the amount of time it takes to calculate everybody's retro pay, confirm the payments and send them off to whereever the cheque's get printed, or in our case, to set them up for the April 15 pay deposit.
Be happy... at customs, we are getting our retro pay not in one lump sum, as in past years, but split over a number of pay periods over the next few months. Which really sucks, since I was hoping to take a trip or something.