An Auckland same-sex de facto couple has won an historic court ruling allowing them to apply to jointly adopt children.

A High Court judge has agreed the phrase "two spouses" used in the 1955 Adoption Act should include same-sex de facto couples, not just those who are legally married.

Lawyer Stewart Dalley said it ends the couple's struggle, which started three years ago, initially before the Human Rights Commission, after a failed bid to jointly adopt their infant daughters.

The ruling set a clear precedent for other same-sex couples, he said.

"In the judgement, it clearly states that there is no justification for same-sex de facto couples, whether they're male or female, not to be granted eligibility status to apply to jointly adopt children."

The adoption process was generally a two-step process, with applicants first needing to meet eligibility criteria, which used to be that they needed to be married - to jointly adopt - or be single, Mr Dalley said.

The second step is the consideration of whether the adoption is in the welfare and best interests of the child.

"What was effectively happening to same-sex de facto couples was that they fell over on the first hurdle because they were never granted eligibility status, so the court was never able to determine whether the adoption was in the best interests and welfare of the children."