Measurements are taken from the actual tee's when lying flat. For the best reference, compare these measurements to a tee you have at home that you like the fit of.
Chest measurement is taken from seam to seam across the chest.
Length measurement is taken from the shoulder seam to the bottom hem.
Shoulder measurement is taken from T-seam on shoulder to other T-seam on the other shoulder.
Use this as a rough guide to compare to AUS sizing:
XS = 6
S = 8
M = 10
L = 12
XL = 14