I'd probably bag them in groups of 4-5 posters. Then tape the bags to a piece of cardboard larger than the bag by 10-12cm in all dimensions. The just put the 4-5 bundles of bagged/boarded posters a box with enough bubble wrap padding so nothing shifts around.
if your sending to australia, screw them up (or fold depending on your preference), wipe your arse and flush down the shitter, it will come down the drains and be here in a week.Thanks,
SHIT yeah