The Catholic church is being investigated by Tasmania’s anti-discrimination commissioner over claims an anti-same-sex-marriage booklet it published was offensive and insulting.

The booklet was distributed in mid-2015 and outlined the church’s stance against changing the Marriage Act to include same-sex couples.

Human rights campaigner and Greens lower house candidate for the 2016 election, Martine Delaney, lodged a complaint with the anti-discrimination commissioner who confirmed on Thursday an investigation would go ahead.

That can lead to the complaint’s dismissal, a move to conciliation or referral to a tribunal.

“I’m more than happy for it to go to conciliation,” Delaney said on Friday.

“I’ve sought an apology and for the Catholic education system to involve itself in LGBTI awareness for students.”

The book warns that “same-sex friendships” are very different from “real marriages” and was distributed to schools around Australia.

A legal rights division of the Institute of Public Affairs (IPA) has criticised the commissioner for going ahead with an investigation.

“If the Catholic church cannot distribute a booklet on Catholic teaching to Catholics, who can it distribute them to?” Legal Rights Project director Simon Breheny said, adding that the complaint should have been dismissed.

The booklet was produced by the Australian Catholic Bishop’s Conference and authorised by Hobart’s Catholic archbishop Julian Porteous for distribution to thousands of Tasmanian families through the church’s school system.

“This case highlights the attack on free speech represented by anti-discrimination law. It should never be a crime to offend a person,” Breheny said.

With a national plebiscite on the issue of same-sex marriage forecast, all perspectives and opinions must be allowed to be heard, he added.

Delaney maintains that the booklet is inappropriate and that it marginalises same-sex couples and their families.

The commission does not comment on matters under investigation.