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Adoption is being used by migrants as a way to "sneak" their relatives into the country, immigration advisers say.
Asian and Pacific migrants were adopting their nephews and nieces with the intention of bringing in their siblings in the future, they said.
One adviser said they were doing so out of "desperation" after the sibling and adult child category was closed in 2012.
The Green Party and the New Zealand Association for Migration and Investment (NZAMI) want the Government to review the policy.
A 48-year-old Tongan mother, who adopted her 11-year-old niece, said she was hoping it would be "a first step" to get her only sister to New Zealand after their mother died.
"We are hoping that when the girl becomes an adult, she will be able to sponsor my sister -- who is her biological birth mother -- here," said the woman, who spoke to the Herald on the condition of anonymity.
NZAMI president June Ranson said: "As the adult sibling policy no longer exists, we are finding families are being divided. The policy was changed more than two years ago and there is a backlog ..."
Green Party immigration spokeswoman Jan Logie said the decision to close the category was a "rushed policy decision" and the Government did not fully assess its implications.
Immigration Minister Michael Woodhouse said yesterday there were no plans to reintroduce the category. "Adults make a choice to move to this country ... [They] should not have an expectation that their adult siblings or adult children can join them if they do not already qualify under existing visa settings."
Tuariki Delamere, an immigration adviser and former immigration minister, described the practice as a scam.
"They adopt their nephews and nieces to give them a better life, and free education comes with it."
Parents who applied for residency for their children under the dependent child category need not indicate if the child was adopted, so Immigration could not say how widespread the practice was.
Labour Party immigration spokesman Dr Rajen Prasad said adoption should not be used as a means to get free schooling in New Zealand.
But Trinidad Ubido, 46, who adopted her brother's daughter from the Philippines in 2009, said it was wrong to "paint everyone with the same brush".
Miss Ubido, a single mother, has been embroiled with Immigration for the past five years to get her adoption of her niece recognised. In May 2012, the High Court declined to grant a declaration that Ms Ubido's Philippines adoption had the same effect as a New Zealand adoption order.
The agency is commencing deportation proceedings against the girl, who is now 12. Acting compliance operations manager Natalie Gardiner said Miss Ubido had exhausted all appeal avenues.
How it can work
A migrant wishes to bring a brother or sister into New Zealand.
The migrant adopts a nephew or niece and brings them into the country.
When the nephew or niece becomes an adult, they sponsor their mother or father, allowing that person to enter the country.
- Lincoln Tan of the New Zealand Herald