Humanist Society of Victoria is compiling a list of parents who's child has been coerced/segregated due to Special Religious Education (aka. Christian Religious Instruction) in state schools. They might point you to someone in NSW doing the same. Perhaps MacArthur Atheists?
This issue of child segregation, of course, isn't confined to non-religious families or even non-christian families. Plus, even some christian families wish to attend to their own family's religious identity. That is, without parachurch interference.
the option to go to another class... all parents were told of this (we weren't)
Apparently some enrolment forms have the "opt-out" checkbox. Any way you can locate your copy?
The form sometimes will tell you the name of the external organisation that state government has been compelled to invite onto school grounds and punch a hole in the teaching week for. ACCESS Ministries (VIC), Scripture Union (QLD), etc. If parents looked up that parachurch website and specifically their MISSION statement, their reaction would match yours when you found SECET.org.au.
We need to "opt-out" of contact with child missionaries in public schools? It's outrageous.