The Government ignored advice from the Ministry of Education and Treasury that the private school Wanganui Collegiate should not become a state integrated school.
Radio New Zealand has obtained information under the Official Information Act that show the ministry told the Government that there were already 1400 empty places in secondary schools in the Wanganui-Rangitikei region and integration would hurt existing state secondary schools in the area.
Integration came with a funding increase from $800,000 a year to more than $3 million.
Education Minister Hekia Parata said last year the school had been working toward integration for more than two years and was one of the top performing schools in the region.
Green Party education spokeswoman Catherine Delahunty said the Government had more than tripled the amount of money it gave to the school while closing schools in Christchurch.