A conscience vote in the lower house of the South Australian Parliament has backed legislation which could allow same-sex couples to adopt children.

Key points: South Australia is one of only two jurisdictions nationwide to not allow same-sex adoption

South Australia is one of only two jurisdictions nationwide to not allow same-sex adoption House of Assembly backed the legislation 27 to 16 in conscience vote

House of Assembly backed the legislation 27 to 16 in conscience vote Nationally, the adoption rate has plummeted over the past 25 years, with just 292 in 2014-15

The state lags behind other jurisdictions in Australia and, with the Northern Territory, is just one of two jurisdictions not to allow same-sex couples to adopt.

Those in favour of reform argue the law needs to be broadened to be more inclusive and ensure the state's most vulnerable children are given the best shot at a safe and loving home.

Education and Child Development Minister Susan Close proposed the new Adoption Act, based on recommendations from an independent review.

She said same-sex couples were just as capable of raising adopted children as their heterosexual counterparts.

"The main criteria for being a successful parent is really wanting to have a child and wanting to do the best for that child and anything else is really a secondary consideration," she said.

"There are same-sex couples who are absolutely dedicated to being brilliant parents and there are heterosexual couples who are, frankly, awful parents."

Dr Close said her state had fallen behind the examples set in other Australian states.

"I've had a lot of support for the same-sex clauses in particular, but when those bells ring and people are required to show up in the chamber and take a side, funny things can happen," she said.

MPs divided on same-sex adoption

MP Susan Close proposed the new changes based on recommendations from an independent review. ( AAP: Ben Macmahon )

The issue has been divisive but passed the House of Assembly on Tuesday evening, 27 to 16, in a conscience vote for MPs.

The Adoption Review Amendment Bill must now go to the Legislative Council.

Labor Government independent MP Martin Hamilton-Smith is an opponent of the bill, saying he believes only a mother-father family structure is appropriate for adoption.

"Children have a right to know who their mother and father is and the best environment for them to grow up in," he said.

"I have no doubt a single parent or a same-sex couple where they to be parents could provide a very loving and nurturing environment for the children, and I'm sure that's occurring in many instances around the country.

"But to me that's not the issue — I think the issue is whether adopted children have the right to have a mother and a father. I think that is the best environment for a child to grow up in."

Number of children in care set to grow

This week's lower house vote coincides with ABC's Four Corners program having exposed glaring shortcomings in residential care, where vulnerable children are left at risk.

Foster carer spokesperson Rachel Titley said the debate is timely, also given it being World Adoption Day.

"We really need to understand that we don't live in a vacuum, we have many children that are being shuffled," she said.

"We have one-in-three children who have been to over 10 placements and we're on a trajectory that nationally we'll have over 400,000 children in care in 20 years' time."

A conscience vote on changes to adoption laws has passed in the SA lower house. ( ABC: Gregory Nelson )

Ms Titley said vulnerable children are entitled to a loving, stable family regardless of their parents' relationship status.

"I think we've just got to understand we're nestled in a community that just need to be in a family, need to out of the uncertainty of the shuffle," she said.

"There's been a massive pendulum swing away from the understanding that families are the best way to care for our kids, especially our most vulnerable."

Nationally, the adoption rate has plummeted over the past 25 years, with just 292 in Australia in 2014-15, according to Australian Institute of Health and Welfare.

The proposed changes also include a greater emphasis on children's rights.

They including the right for children to keep their original name and that the adoption of adults also be allowed.