An Auckland school paid nearly $20,000 to send 12 teachers to the Cook Islands for two weeks.
The Auditor-General is probing the spending by Blockhouse Bay Intermediate, which saw teachers visit the archipelago - including the tourist hotspot of Aitutaki - during the July school holidays last year.
The school is defending the spending, saying it was "very much a working trip and not a holiday".
Its purpose was to immerse staff in Cook Islands culture and teaching practices to ensure Blockhouse Bay Intermediate's teaching practices benefited Pasifika pupils' learning, principal Michael Malins said.
But one parent said it was a "crazy" use of money that could have been better spent on resources.
The Auditor-General considers overseas travel "sensitive expenditure" with potential to provide a private benefit to a staff member.
"We consider it unusual for schools to fund overseas trips for staff and students. Our auditor will be making enquiries about this trip during [an upcoming] audit," a spokesman said.
Blockhouse Bay Intermediate's associate principal planned the Cook Islands trip.
The school wanted to challenge perceptions that Pasifika pupils' cultural background was a barrier to educational achievement, Malins said.
Before leaving Auckland the 12 teachers - a fifth of the staff - attended professional development courses about raising student achievement and progressing the Ministry of Education's Pasifika Education Plan.
According to an Education Review Office's 2016 report, pupils of Cook Island ethnicity made up about 1 per cent of the Blockhouse Bay Intermediate's roll. Other Pasifika peoples made up 6 per cent.
The performance of Pasifika pupils at the school was below that of their Asian and Pakeha peers, but above Māori children in national standards writing tests in mid-2017.
The teachers who went on the trip were welcomed by Cook Islands Ministry of Education officials when they arrived on July 8.
They spent most mornings observing in classrooms or churches and afternoons on "de-briefing/reflection".
The itinerary for the afternoon of July 15 is blank. That morning the group had flown to Aitutaki, a popular honeymooners' destination. They left the Cook Islands on July 21.
The school required the teachers to document their experience and present findings to staff when they returned.
When asked what the tangible benefits of the trip were, Malins said progress of Pasifika students in writing "strengthened significantly".
Spending time in a Pacific Island nation had increased teachers' cultural awareness, which benefited all students, he said.
The school paid $18,976 for the 12 teachers' flights and insurances, documents obtained by the Herald on Sunday under the Official Information Act show.
Teachers covered their own accommodation costs and all other expenses.
The money the school used was from international student fees, Malins said.
A spokesman for the Auditor-General told the Herald on Sunday unless a school fundraised for a specific purpose it viewed any funds a school received as public money.
It would investigate Blockhouse Bay Intermediate's spending on the trip during its next round of audits in April and May.
A mother of a pupil, who asked not to be identified, told the Herald on Sunday the school hadn't been transparent about the trip.
"There was absolutely nothing within the newsletters. If it's something that they're genuinely being proactive in, then I would have thought they would have done some sort of presentation to the Pasifika group at school."
Engaging more closely with local Pasifika communities would have been more beneficial to raising the achievement, she said.
The woman, who works in the education sector, found out about the trip through an acquaintance closely connected to the school.
"It was a real shock. It's such a lot of money. It's just crazy. They should have to give the money back to the Ministry of Education because that's not money well spent," she said.
She believed the funds would have been better spent on resources, like books or laptops.
The school attracted the attention of the Auditor-General in December last year after it did not immediately pass on thousands of dollars it had raised for Fiji flood victims during 2016.
The mishandling happened before the current principal or board of trustees took office, but Malins confirmed this week the money was sent to the Red Cross in February.
MInistry deputy secretary Katrina Caseysaid although spending on overseas travel by schools could sometimes be justified, "the threshold is high".
Guidelines for school boards stated overseas travel must be considered against competing priorities, such as teaching. Casey said the ministry knew Blockhouse Bay Intermediate travel plans several months before the trip. It was mentioned "in passing" to a ministry representative during a meeting about Pasifika students.
She would not be drawn on whether the ministry considered the trip appropriate.