A man who donated his sperm to a lesbian woman so that she could have a child will argue in the High Court today that he is legally the girl's parent and should be able to object to her moving overseas.

Key points: Court action began after girl's mothers wanted to move to New Zealand

Court action began after girl's mothers wanted to move to New Zealand The man donated his sperm on the basis he would be involved in the girl's life

The man donated his sperm on the basis he would be involved in the girl's life Mothers could not prove they were in de facto relationship at time of girl's birth

The man, who the court referred to by the pseudonym Robert, fathered the girl in 2006 via artificial insemination with the understanding he would be involved in her upbringing.

However the girl's mother, Susan, and her wife, Margaret, decided to move to New Zealand, triggering Robert's bid to keep them and the girl in Australia.

Documents submitted to the court showed Susan had since had a second child, who was not related to Robert. But submissions to the court claimed he had been involved in the younger girl's life too, and she had only recently learned he was not her biological father.

Lawyers for Robert said both girls referred to him as "daddy", and he was listed as the older girl's father on her birth certificate.

The man was listed on the girl's birth certificate as her biological father. ( Pixabay )

They said he had had a big role in her upbringing, volunteered at her school's canteen, and that both girls had an involvement with his extended family.

After an initial Family Court ruling, Susan and Margaret were restrained from establishing a home in New Zealand with the two girls.

Part of the finding was that, at the time the older child was conceived, the women could not prove they were in a de facto relationship.

The conditions of the ruling included that the two women were to be given equal parental responsibility, but that they must consult the man about long-term decisions.

Biology 'not the whole answer'

In making her decision, Family Court Justice Margaret Cleary acknowledged the strain the dispute was having, particularly on the long-standing relationship between the girl's biological parents.

"There is interpersonal bitterness, there are accusations by the two women of manipulative, overbearing conduct by [Robert] and accusations by [Robert] of ruthless exclusion by the two women," she said.

Justice Cleary said the law recognised parents in different ways.

"Being a biological parent is not the whole answer to the question 'who is a parent?'," she said.

"However, where there is a challenge to a biological parent being a legal parent, as there is here, biology is a part of the answer."

Justice Cleary ultimately found the man was a parent, because he took part in the artificial insemination in the belief he would have a role in the girl's life.

She also pointed to the fact Susan and Margaret could not prove they were in a de facto relationship at the time she was conceived.

But that decision was overturned by an appeal to the Family Court, partly based on a New South Wales law governing fertilisation procedures, specifically providing for same-sex couples.

Robert had also formed a relationship with the couple's younger daughter, who was not related to him. ( Unsplash: Annie Spratt )

The women argued at the time that, as a sperm donor, Robert could only be legally treated as a parent if he had been married to Susan at the time of conception.

And the case also involved a constitutional issue, because both sides were putting arguments about whether there was an inconsistency between a state law and Commonwealth law.

Normally a Commonwealth law would take precedence, but the couple's case has been backed by the Victorian Attorney General who said the New South Wales law applied by itself and the man was not the girl's legal parent.

The Federal Government has intervened to support the man, saying he was legally the child's father under the Family Law Act.

The High Court will begin hearing arguments today.