From what I can tell from an admittedly brief search for more information is that if the federal government was to federally criminalise conversion therapy a legal challenge could be mounted by state governments as federal criminal law normally in relation to crime that crosses state borders (i.e. drug trafficking) and might be seen as federal overreach. (Though during my reading apparently the federal government has overridden state laws before; namely overriding Tasmanian sodomy laws so maybe it could be done)

Private school funding on the other hand I'm not as certain about. I don't know if the Act that details how funding for non-government schools is dispensed allows the federal government to place restrictions on funding based on discriminatory practices in these schools. It might be something that needs to be done on a state level.