As far as CASA is concerned, there is no cutoff for myopic error. Their regulation is:
An applicant must have a distant visual acuity of 6/9 or better in each eye separately (6/12 for Class 2) and 6/6 or better binocular (whether with or without correcting lenses) (6/9 for Class 2).
They also go on to state:
CASA has not placed restrictions on applicants who require high levels of correction in order to meet the required visual standards. CASA considers that ability to meet the standard is all that is required, regardless of the power of corrective lenses necessary to achieve this outcome.