Education Ministry figures show a quarter of Auckland's 503 schools had enrolled more students than their official classroom capacity by the middle of the year.
The 130 schools exceeded their permanent classroom space by 11,750 students, with classroom utilisation rates ranging from 101 to 180 percent.
Several schools had hundreds of students more than their official roll capacity and one, Mount Albert Grammar School, had more than 1000.
The figures provided to RNZ followed mid-year roll returns that showed Auckland schools had experienced their fastest growth in 17 years with 4113 more students than at the same time last year.
They also come as the ministry moves to introduce or amend enrolment zones at 135 Auckland schools.
However, the ministry said only 28 of the 130 schools that exceeded their capacity were definitely getting more classrooms.
It said 50 needed to take fewer children from outside their enrolment zones and the remaining schools were being considered for more classrooms and/or for new or amended enrolment zones.
The ministry said its calculations did not include temporary classrooms, rooms schools had purchased themselves, or teaching spaces smaller than 40 metres square so the figures did not necessarily mean the schools were over-crowded.
Pōkeno School was, on paper, the most over-capacity of the region's schools at 180 percent utilisation. In July it had 362 children and formal classroom space for 201.
The school's principal, Blair Johnston, said that was a challenge, but relief had since arrived.
"We've currently got 391 and in comparison to our capacity we're actually okay right now, but that's because we've had an additional five temporary classrooms put on our site," he said.
"We were certainly well over capacity earlier this year."
Primary schools usually start each year with fewer students than they finished the previous year, but Johnston said Pōkeno was in the unusual position of expecting to start next 2021 with more students than it finished 2020 with.
Among secondary schools, Mount Albert Grammar, was 49 percent or more than 1000 students over capacity, with 3128 students.
The school's principal, Patrick Drumm, said the school could cope because it had 48 temporary classrooms - the equivalent of several primary schools.
"While the numbers look a bit frightening in terms of 149 percent capacity, every student is in a classroom because there are temporary classrooms which the ministry provided, and also topped up with school-owned building as well," he said.
The ministry's figures said the school needed to reduce its out-of-zone enrolments and Drumm said it had been doing that for the past four years.
He said about 15 percent of Mount Albert Grammar School's enrolments were out-of-zone and that would drop to 5 percent in five years.
Even with that change, the school was expected to grow. It would get 60 new classrooms over the next 10 years and some forecasts suggested it could reach 4000-4500 students.
"If you drive around our local community, you blink and there's another development under way and so you get a feeling that there's some reality to those projections because we're losing single-dwelling properties and suddenly there's multi-level apartments that will house many, many families, and potentially students that will need to go to schools," he said.
The ministry's figures said Macleans College was 40 percent over capacity with 2626 students and it needed to reduce its out-of-zone enrolments.
The school's principal, Steve Hargreaves, said the ministry had not raised that with the school.
He said last year the school accepted only the siblings of existing students as out-of-zone students for 2020 and this year it had about 500 out-of-zone students.
Hargreaves said it was likely out-of-zone enrolments would become a thing of the past right across the city.
"We have all this in-fill housing happening inside our zone and what that means is that in a few years time we'll only be able to accommodate the in-zone students," he said.
"Those schools with a big housing development next door that currently don't have a zone, they will probably have to get a zone very smartly and that's going to change the whole dynamic of students electing to travel across town to go to school. I think it's probably going to become a thing of the past in a decade or two."
Hargreaves said the school had built its own classrooms so it was not over-crowded.
"We're quite full, but we have spare classrooms every period, so we're not in a position where students don't have classrooms with desks and the appropriate equipment in them," he said.
Though many Auckland schools were struggling with rapid growth, others had spare capacity and the ministry's figures showed there was room for 30,417 more students in the region's classrooms.