The Australian Border Force has taken its first New Zealand youth into immigration detention.
Its latest figures from March shows a boy, of unknown age but under 18, was among 200 New Zealanders being held.
He was one of just two foreign youths being held in detention.
Border Force figures do not show where the youth was being held or if he is still there.
Prime Minister Jacinda Ardern said she was concerned to hear of the report.
"We have expectations within New Zealand around the way that we deal with minors in those scenarios.
"Obviously, when it comes to other jurisdictions, we would be interested if they were indeed New Zealand citizens," she said.
She had not had an individual briefing on the case and would be seeking further information.
Most New Zealanders are held after their visas have been cancelled.
They face deportation if they do not continue to pass Australia's character test, most commonly for being jailed for more than a year.
The statistics also showed that the number of New Zealanders being detained, including 17 women, were at their highest level for almost two years.
Fourteen percent of all detainees in Australian immigration centres were from New Zealand.