Scrap that, by your measurements, one may or may not fit a 3k satchel but would easily in a 5kg safely boxed,  and a B2 is too small, but a B4 might work.  So sending two smaller parcels would be a little cheaper than both together.

Another way is box them and purchase one of the largest size Auspost bags, the post office doesn`t cube them, its by weight only, and it is much cheaper.