Family First appealed against its second deregistration, but the High Court dismissed the appeal. That decision was then reversed by the Court of Appeal, which found the organisation did qualify as a charity.
The Supreme Court has now overturned that decision.
A summary released by the court said expressing a viewpoint was not an automatic disqualifier, but charities must genuinely seek to educate rather than advocate and do so in an objective and balanced way.
"Applying that test, the Court found that Family First's purpose [exhibited by its trust deed and activities] crossed the line between education and advocacy. Its research reports lacked the balance that is required to further an educative purpose," the summary read.
The organisation also did not qualify on the grounds of benefits to the general community.
"It advocated a particular version of the family, being the traditional man-woman marriage. Its purposes are discriminatory - it advocates for measures to prefer the traditional family to the disadvantage of others."
The Court said Family First's engagement on abortion, assisted dying, prostitution, and censorship did not necessarily support its wider purpose of advocating for marriage and family.
Instead, it said these were free-standing political objects on which people held differing views and said it could not determine whether the views promoted were publicly beneficial or otherwise charitable.
In addition to the judges' joint findings, Justice Joe Williams noted that since at least 1805 the law had avoided hard-and-fast rules for identifying charitable purposes, which had resulted in inconsistencies.
To help guide future decision-making, he suggested selflessness could be used as a touchstone.
"Substantially self-regarding purposes should not, in principle, be charitable," the court's summary read.
He noted Family First's promotion was self-referential and was not about community.
"Family First's advocacy is not fair, balanced or respectful, so its advocacy is not charitable."
