At other times, parents have listed their children at addresses of friends or distant family, or moved into rental housing in a desired school zone just for a few months during the enrolment period.
Auckland Grammar headmaster Tim O'Connor said hiring a private investigator was a last resort. But there was huge pressure for places at the school.
"It almost seems like every year the pressure on our roll is unprecedented," Mr O'Connor said.
A fulltime enrolment officer at the school, a position created in 2006, had been unable to cope with all the necessary checks alone, and brought in outside help.
"If you saw the process you would say it's being overly pedantic, but it's about compliance," he said.
Parents were notified before the visits - "it's all done in a convivial manner".
Rangitoto College principal David Hodge said his school had one or two cases every year where students were evicted because their parents had supplied false information.
"If it's three or six months down the track, that's massively disruptive for the kids. It's a high-stakes game. The parents do it because they want the very best for their children, but if they get caught their children are likely to suffer."
Some cases were easy to discover - such as a family residential address pointing to an empty section. For other cases, enrolment administrators had become attuned to spotting suspicious applications, Mr Hodge said.
"Ninety-nine point nine per cent of parents are absolutely straight up and honest and brilliant people.
"[Though] we don't know about the ones who get away with it."
Secondary Principals Association vice-president Paul Daley said schools had to be scrupulous to be fair to all applicants.
"It's certainly not fair on the schools, but that's the reality."
There could not be a more severe penalty than evicting students, he said.
"If anything needs to be done, it's that the Ministry of Education supports the schools in carrying out the requirements of the enrolment schemes."
Association president Patrick Walsh said there were no restrictions on schools making checks.
Some schools required rates bills to prove parents owned their listed addresses.
Difficulties arose when trying to evict already enrolled students, he said.
There had been legal victories for parents pushing the boundaries. Parents who had rented in a school zone only during enrolment, moving out in a few months, had won appeals against their children's eviction.
"The onus is on the school to prove that the intention hadn't been to remain living there - and it's very difficult to prove."
In the background was intensifying competition in education, he said.
"Parents are realising you need to get your children in high quality [secondary] schools to get into university programmes which are increasingly restricted," he said.
"People will do all kinds of strange and bizarre - and at times unlawful - things to get their kids in a school."
School Trustees Association president Lorraine Kerr said it was unfortunate that schools were having to go to such lengths to investigate parents, though there were always varying circumstances.
