The amount parents paid in school donations jumped more than $23 million between 2008 and 2010 - a 31 per cent increase over a time in which Government funding for schools increased by 8 per cent.
Ministry of Education figures show parents paid a total of $98.4 million in donations to schools in 2010 - up from $75 million in 2008.
The figures were obtained by Labour, which said donations increased by only 6 per cent over the six years preceding 2008, when it was in Government.
The sharpest increases in donations were in mid-decile schools - the total raised in donations by decile seven schools increased by 56 per cent, and the increase in decile four schools was 46 per cent.
The only decile in which less was received was decile one schools, where the amount had dropped by 7 per cent.
Labour leader David Shearer said it was likely that was because of parents' inability to pay.
The donations have long been controversial - although they are legally voluntary, schools have taken steps such as calling in debt collectors to follow up on non-paying parents. Other schools have imposed sanctions on students.
Last year Macleans College, in Auckland, made students pay extra for extra-curricular activities - including $50 more for a ball ticket - if their parents had not paid the $460 donation.
Donations make up only a portion of the total income schools get from parents and their local communities. Overall in 2010, parents paid $267 million in donations and compulsory charges.
Between 2008 and 2010, the total amount gathered in locally raised funds increased by 5 per cent, but the donations portion of that increased by 31 per cent - indicating fundraising was a struggle in the economic downturn.
Mr Shearer said the figures showed the cost of running schools and providing quality education was outstripping the Government's willingness to pay for it. "School operating budgets are clearly too low, and they're passing that on to parents," he said.
Over the same period the amount schools received in Government grants went up by 8 per cent, from $5.2 billion to $5.6 billion.
That increase was far slower than in 2006 to 2008, when Government grants increased by 32 per cent.
Asked if she was concerned about the increase in donations, Education Minister Hekia Parata said it was up to boards of trustees to set donations. She said the Government had increased funding for education every year to $9.6 billion this year.
Overall, schools rely on income from donations, charges for activities and fundraising for about 10 per cent of their total revenue - a proportion that has barely shifted since 2006.
Secondary teachers' union the PPTA will discuss the reliance on parents and fundraising at its annual conference this month.