Boom Boom!
Nice wall Phil
Unfortunately you point out something all too frequent. The main pressure concern behind the wall is not the "weight" of the soil it is holding up, but the hydrostatic pressure that builds up during wet weather. Inadequate drainage and insufficient footing/posthole preperations are the leading causes of retaining wall collapse with incorrect material selecting a close third.
I don't know if it applies in Rogers case in WA, but in the council shire I live in (and SE qld generally) you are not permitted to build a retaining wall more than 990mm high (in a domestic situation) without submitting engineering plans to council.