Labour says the Government is "way behind" on education spending, and as a result, some schools are so overcrowded children are learning in halls and former dental clinics.
In 2014, National announced $350 million for new classrooms and schools in Auckland, to be spent over four years.
So far, the government has spent only $107 million of that.
A Ministry of Education report found 214 New Zealand schools are overcrowded, with an additional 488 schools at risk of overcrowding; of those, 120 overcrowded schools are in Auckland.
Labour is blaming that on a lack of spending.
Speaking on The AM Show, Labour Party leader Andrew Little said the Government needs to get ahead with building new classrooms.
"Kids [are] learning in school halls, in former dental surgeries at schools. Totally unacceptable.
"The Government said in 2013 they had a plan to fix this. They put $350m aside. They spent about $100m. Just way behind. If you're gonna tell the people you're going to do something about it, you've got to get on and do it."
When answering a question in Parliament in March, then-Associate Education Minister Nikki Kaye said the ministry had spent approximately $107 million of the $350 million earmarked for overcrowded Auckland schools - but she said a total of $300 million had been "committed to projects".
Ms Kaye said she expected the fund to be exhausted in the 2019/20 financial year.
With the funding announced in 2014 to be spent over four years, that could end up being two years behind schedule.
Mr Little said until the Government releases the Budget, Labour wouldn't be able to come up with their own costed plan
"Once we know what those figures are with a degree of accuracy then we can get out our detailed plan about it," he said.
In April, Mr Little said Labour would reduce immigration by tens of thousands, with the cut coming from work visa and student visa schemes.
- We'll cut tens of thousands of immigrants - Little
- Thousands of student visas would be cut under Labour govt - Little
At the time, he singled out immigrants coming in as labourers on work visas.
"Six thousand work visas are issued for people to do labouring work and we've got 15,000 out-of-work labourers. That doesn't make sense," he said.
But with all this building of classrooms - and don't forget those 100,000 affordable homes promised through Labour's Kiwibuild - could those labourers coming in on work visas be sorely needed on building sites across the country?
"You can manage immigration better, keep the pressure off the big cities [while] at the same time making sure the people we do let in have got the skills that we need to do that stuff that's going to add value - build homes, build schools, build cities and do good things for New Zealand," he said.