IIRC it was a licence obligation applied to commercial broadcasters. Some stations had religious programming while others relied on more frequent PSAs. 2SM, being owned by the Catholic Church, even at its peak 70s rock era, broadcast Mass every Sunday night. 3XY in the 80s had Sunday night talkback/counselling with Rev Tom Stokes. 3UZ/RSN still has Father Dowling, who this week celebrates 50 years on Sunday nights (though some of that 50 years was at 3DB)
(RSN kindly filing this story under “greyhound/news”! 
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