A Kiwi international sportsman is mired in a transtasman custody battle after his ex-partner allegedly went on holiday with their child and never returned.

The father, a professional sporting star, who along with the others involved in the case cannot be named because of Family Court laws, has invoked the international child abduction agreement, the Hague Convention, to be reunited with his child.

A Family Court judge in New Zealand has ruled that the child was wrongfully removed from Australia.

Judge Catriona Doyle last month ordered that the young child return to Australia within 21 days.

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But the mother has appealed the decision to the High Court.

The parents, both New Zealand citizens, moved to Australia about two years into their relationship and took children from previous relationships with them, according to Judge Doyle's written decision seen by the Herald.

They separated about two years after moving to Australia and the father was not aware his ex-partner was pregnant, Judge Doyle was told.

The child was born prematurely later that year. The mother says she hadn't planned for the child to be born, or remain, in Australia.

The father only found out about the birth through a mutual friend, the court heard.

After the birth, the baby and mother returned to live with the father, who was in a new relationship.

Early last year, the mother wanted to return to New Zealand with the child.

In text messages, the father agreed, as long as it was only for a holiday and not a permanent move.

"The text messages from the mother to the father […] show, at times, the mother's clear intention was to return permanently to New Zealand, but also appear to indicate she remained committed to living in Australia to see out her dreams of having a small business and because of the greater opportunities for the child in Australia," the Family Court decision says.

The court heard that the mother got travel documentation without the father knowing and left the country.

He only found out they were in New Zealand after recognising a park in the background of a photograph.

The father later lodged Family Court proceedings, invoking the Hague Convention, claiming the child was wrongfully removed from Australia to New Zealand and that, at the time of the child's removal, they were habitually resident in Australia.

The mother denied the claims.

Judge Doyle said an unusual aspect of the case was that the parents separated before the father, and possibly the mother too, was aware of the pregnancy.

If the child had been born on their due date, the mother says they would have already been in New Zealand, and if that had happened, the father would not have been able to call on the Hague Convention.

"Counsel were not aware of, and I have not been able to find, any other cases with an analogous fact scenario," the judge noted.

However, she ruled in favour of the father, concluding that the child had been born in Australia and had lived there for their entire life before being removed.

"The father has therefore established the grounds in [Family Court law] and the court must make an order for return of the child, notwithstanding the length of time she has been in New Zealand and the likely disruption to her a result [sic] of the making of that order."

Yesterday, the father spoke of his heartbreak at being separated from his child.

"Seeing [the child] every day, we'd formed that special bond. It's been so tough," he told the Herald.

"[The child] needs to come back here, which is home. Like a lot of Kiwis, we came here for a better lifestyle, better opportunities, and I want that for my [child] as well."

The father said if the mother does not return to Australia with the child, he will travel to New Zealand to get them.

If the mother is prepared to go back to Australia, the father assumes the child will live primarily with her "pending determination of care arrangements" through the Australia Family Court, and seek contact as often as possible. He's offered to pay for their flights.

The mother has appealed to the High Court, claiming the decision is wrong and contains errors of fact and law. It's been filed out of time but she has applied for a time extension.