Gay couples in Tasmania will be able to adopt children after the state's Upper House backed a changed to the law.

Under the new law, couples who have a relationship registered under the Relationships Act can adopt children not known to them.

The bill received the support of all but three MPs, with independents Ivan Dean, Rosemary Armitage and Liberal Leonie Hiscutt opposing it.

The Government's only Upper House MP, Craig Farrell, told Parliament the change removes unfair discrimination against gay parents.

"This bill doesn't say same-sex couples are to be considered over any others," Mr Farrell said.

"It gives them the ability to apply, and it is then up to the authorities to say what is in the best interests of the child.

"It is really a pretty straightforward piece of legislation."

Liberal Independent MP Tony Mulder said the change makes adoption fairer for parents and children.

"I see no detriment to a child," he said.

Murchison Independent Ruth Forrest believed the change was in the best interests of children.

"The idea that gay parents are second best must be challenged."

"To suggest that a same-sex couple is not as able to raise a child as a heterosexual couple, is both absurd and unsubstantiated.

"Discouraging gay adoptions is severely diminishing loving, stable homes for the children who are waiting," she said.

Independent MP Ivan Dean disagreed saying children had a right to start life with a mother and father.

Leonie Hiscutt also opposed the bill.

"I can't see the need to change this bill for the betterment of children, I think that children are well-served with the existing arrangements."

"I can only see that this bill fulfils other purposes which should be addressed elsewhere."

'Full circle' for gay rights

The law had passed through the House of Assembly in April after the Liberal party allowed a conscience vote on the issue.

Gay and Lesbian Rights Group spokesman Rodney Croome has applauded the law change

Mr Croome says the change shows Tasmania has come full circle, after being the last Australian state to decriminalise homosexuality during the 1990s.

He says the change will allow foster children to be adopted by same-sex couples when judged to be in the child's best interest.

"Today is a proud day for Tasmania, for we have removed the very last vestige of discrimination against same-sex couples in existing state law."

"The passage of the Adoption Bill gives me hope that we will revisit the marriage bill and pass this, the open heartedness of some MLCs to do what's in the best interest of children, removing discrimination against same sex couples, having a more tolerant and inclusive society," he said.

The Save Marriage Coalition, which opposes the bill, has criticised the law for being too adult-focussed.

The organisation's spokesman Guy Barnett says "the bill will relegate some Tasmanian children to a life without mothers or a life without fathers".

Mr Barnett says the law is a step towards marriage rights for gay couples.

"In some ways it does send a message that the legislators are considering the merit of same-sex marriage but that is also another debate for another day and I hope the silent majority in the community will express their views accordingly," he said.