Typically the private schools in WA have a broad range of criteria when looking at school enrolments. Things like whether the child is the same religion (proof of baptism, first holy communion, confirmation etc required), whether either of the child's parents are an 'old boy' or 'old girl' of the school, how long the child's name has been on the waiting list, etc. If I were you I'd email the schools you're interested in and ask - alternatively, check their websites (if they have them - most private schools do) to see if this info is on there.