The Catholic Church has responded to moves to legalise same-sex marriage in Australia with a letter which is being sent to parishes around Tasmania.

The letter is titled "a pastoral letter from the Catholic Bishops of Australia to all Australians on the 'same-sex marriage' debate" and carries the slogan "don't mess with marriage".

It was compiled by the national body the Australian Catholic Bishop's conference and is not specific to Tasmania.

The same booklet has been distributed to Catholic school children in Canberra.

Earlier this month Opposition Leader Bill Shorten introduced a same-sex marriage bill into Federal Parliament which would replace the word's "man and woman" with the term "two people" to define who can be legally married.

A number of MPs on both sides of politics came out in support of changing the definition of marriage to allow same-sex unions.

A few days later, a South Australian Bishop warned children of gay couples will see themselves as another Stolen Generation because they have been denied a mother and a father.

Tasmanian Archbishop Julian Porteous said the letter was issued to help Catholics understand why the church was opposed to changing the legal definition of marriage.

"This Pastoral Letter is offered to parishioners in the hope that it will deepen their appreciation of the beauty and dignity of marriage and family life according to the plan of God," he said in a statement issued on Wednesday evening.

The small booklet begins with a section titled "respect for all" and then goes on to outline why marriage should be exclusively between a man and a woman.

We now face a struggle for the very soul of marriage. Only women are admitted to women's hospitals and only children to primary schools. We have programmes targeted at Aborigines, refugees, athletes, those with disabilities or reading difficulties, and so on. Thus privileging or assisting particular people in relevant ways is not arbitrary but an entirely fair response. And if the union of a man and a woman is different from other unions, not the same as other unions, then justice demands that we treat that union accordingly.

Advocates for same-sex marriage were quick to brand the church as "out of touch".

Church argues it is 'gravely unjust' to: Legitimise the false assertion that there is nothing distinctive about a man and a woman, a father or a mother

Legitimise the false assertion that there is nothing distinctive about a man and a woman, a father or a mother Ignore the particular values that real marriage serves

Ignore the particular values that real marriage serves Ignore the importance for children of having, as far as possible, a mum and a dad, committed to them and to each other for the long haul

Ignore the importance for children of having, as far as possible, a mum and a dad, committed to them and to each other for the long haul Destabilize marriage further at a time when it is already under considerable pressure

Destabilize marriage further at a time when it is already under considerable pressure Change retrospectively the basis upon which all existing married couples got married Source: Pastoral letter on the same sex-marriage debate

Australian Marriage Equality national director Rodney Croome said the church had every right to hold its own religious definition of marriage.

"But the Marriage Act is a civil law which does not and should not enshrine the values of any particular religion," he said.

Mr Croome said his group did not wish to force the Church or any other group to do anything it did not want to do.

"The Catholic Church will not be forced to marry same-sex couples and in return I ask it not to force its views on the rest of the community through the Marriage Act," he said.

Mr Croome said most Australians were in support of allowing same-sex marriage in the country and the church hierarchy was "out of touch with it's own flock".

"Polling by national research company Crosby/Textor in August 2014 found that 67 per cent of Australian Catholics support same-sex marriage," he said.