KidsCan's waitlist is at record levels with 10,000 children, in 260 schools and early childhood centres, needing help.
The charity, which provides food and clothing to schools and early childhood centres, said the list was the longest it had ever been in its 19 year history
Chief executive Julie Chapman said the repercussions were huge.
"The poverty line is shifting. It's heartbreaking to see children who didn't need help before now arriving at school lacking the very basics, and we are unable to get food and clothing to them because we don't have enough funding. That's why we're launching an urgent appeal," she said.
KidsCan said schools were expressing more concern to them about students' living conditions as winter bites.
One social worker described visiting homes to find up to 30 people living together, or multiple tents on the lawn, it said.
It said the majority of the 98 schools on its waiting list were in middle-income areas, and 56 of those schools did not qualify for the government's Ka Ora, Ka Ako lunch scheme.
KidsCan was launching an urgent appeal to reach children waiting for help, backed by its principal partner Meridian Energy.
"We can't leave children waiting for support at such a critical time in their development," Chapman said.
Associate education minister David Seymour said he understood the challenges people were facing.
"Issues around cost of living are affecting more and more people, many who wouldn't consider themselves vulnerable.
"That's why this government is doing more, including extending school lunches to provide meals to 10,000 more children in early childhood education, in some of the country's most hard-pressed areas.
"We're also doing the Healthy School Lunch programme better and smarter, for over $100 million less, taking pressure off inflation and interest rates so that people can afford to provide for their children."
Meanwhile, Seymour was inviting food manufacturers, producers, wholesalers, and distributors to register their interest in the revamped Healthy School Lunches programme.
The initiative was re-designed earlier this year and will launch in term one of 2025.
"The Healthy School Lunches programme delivers lunches to over 240,000 children. It is a great opportunity for enterprises in the food industry to be a part of one of the biggest food programmes in the country," Seymour said.
"The next step in the process is for enterprises with existing food infrastructures across communities to register their interest and pitch for being part of a more efficient Healthy School Lunches programme."
Registration of interest in the Healthy School Lunches programme would be open on the Government Electronic Tender Service website from 12 August.