Academic marks don't seem to make a big difference as suggested above. I got through after only having completed year 11 (I later completed Y12 through the Army) and passed the testing well.
The requirements are constantly changing, however, due to economic circumstances. When times are good many people don't want a bar of the ADF and so entry requirements are lowered. When times are bad the ADF often becomes an employer of choice 'by default' and standards are raised.