Titled Don't Mess With Marriage and distributed by the Australian Catholic Bishops Conference (ACBA), the 17-page booklet warns against extending marriage rights to same-sex couples.

In a media release, ACBA said marriage equality would make marriage about "intensity of emotion" rather than "a union founded on sexual complementarity and potential fertility".

"Their physical, spiritual, psychological and sexual differences show [men and women] are meant for each other, their union makes them whole, and through their union 'in one flesh' they together beget children who are 'flesh of their flesh'," the booklet reads.

"They share the sameness of humanity but enjoy the difference of their masculinity and femininity, being husband and wife, paternity and maternity."

While it is unknown how many schools across Australia have distributed the booklet, at least 56 schools in the Canberra-Goulburn area, along with others in Melbourne and the Northern Territory, have given it out.

"The pastoral letter on same-sex marriage was distributed to each Catholic bishop across Australia," an ACBA spokeswoman told BuzzFeed News.

"It is at the discretion of each bishop as to what extent the pastoral letter is distributed within their local diocese."