Schools are converting libraries, halls and common rooms into classrooms as the school building programme falls behind New Zealand's burgeoning population.
Education Minister Chris Hipkins has told National education spokeswoman Nikki Kaye that 508 state and integrated schools - more than a fifth of the national total - are now "over-capacity".
Hipkins says spending on school buildings is "on track to eclipse $1 billion" in the current financial year to June.
But school principals say the new buildings, including a surge of new "modular transportable buildings" or prefabs, have been held up by the Labour Government putting a hold on projects planned by the former National Government while Hipkins developed his own plan.
A "national education growth plan" was finally unveiled last July, including 30 new schools in Auckland and 30 in the rest of the country by 2030.
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Waikato Principals' Association president Hamish Fenemor said National promised 200 more prefabs over two years, but that was stopped after Labour took power in 2017.
Then late last year, he said, the Ministry of Education entered contracts with two more companies to build more prefabs in addition to the one established contractor, Interlink Modular.
"Over the last two years things were put on hold while they worked out what was the best way to move forward," he said.
"There seems to have been a change in the last six to eight months and they are now throwing classrooms into schools where there is growth or not from within their own zones. It's weird what's going on, there's no rhyme or reason as to where new classrooms are being built."
Auckland Primary Principals Association president Heath McNeil said his school, Ormiston Primary, was built for 720 students just five years ago but its roll hit 860 at the end of last year and was likely to top 1000 this year.
He has moved 200 children into the adjoining Ormiston Junior College, and last week he decommissioned the school library to accommodate another class. He will also need an extra prefab before the end of the year.
He has moved the school's library books into a breakout space in the junior college.
"There still will be a library. There just won't be room for them to sit and read. They will have to take the books and go back to their regular classrooms," he said.
Auckland Secondary Principals Association president Richard Dykes said his school, Glendowie College, was using its school hall for some classes because it had no spare rooms left.
"We had a students' common room. We can't have that any more," he said.
"We are currently building an 11-classroom block, we'll get a net gain of 10 classrooms by 2021. At the end of that we'll still be over-capacity, but instead of being 130 per cent of capacity we'll be down to 110 per cent."
He said some secondary schools in high-growth areas had been forced to build on their playing fields, leaving some with only one or two fields for school rolls of 1000 to 2000.
Fenemor said his school, Cambridge East School, had capacity for 426 students but would pass 500 this year because of an inflow of families from Auckland and other areas to take up jobs in new Cambridge industrial developments.
Kaye said the Labour Government had failed to keep up with the need for education infrastructure, pointing to a $176 million underspend on the education capital budget in the year to last June.
"There is now a plan for all of New Zealand out to 2030, and $1.2 billion was allocated for the first tranche of that in Budget 2019 – New Zealand's biggest ever investment school property in one Budget," he said.
"When it comes to spending money on school buildings, December 2019 was a record-setting month for the ministry's capital expenditure.... If spending continues at this rate, we are on track to eclipse $1b by the end of the financial year."