More than 100 Auckland families are on a waiting list to receive curtains which could take up to two-and-a-half months, Habitat for Humanity says.
"It's about 1260 curtains that we have to make just to keep up with the demand and we're getting people through every day," Auckland general manager Conrad LaPointe told Newshub.
"We have four people who work permanently within the curtain bank and then we have a whole load of volunteers who help us as well. They do all sorts from sorting to cutting to sewing and making sure the curtains get to the right places."
The need is extreme, Mr LaPointe said.
"On average, we're doing 12 curtains per household to ensure these places are kept warm in the winter."
It's the most demand the organisation has experienced, he said.
And because many people live in overcrowded homes, they are also putting curtains in lounges where people sleep, he said.
To be eligible, families had to either have a Community Services Card or sick children.
Tash and her family, who Newshub has agreed not to name, are on Habitat for Humanity's waiting list.
She has lived in her Ōtāhuhu home with other family members for two-and-a-half years - the whole time without curtains, she said.
So every evening she pegs up sheets over the netting in each room.
"I just kind of double-sheet everything, just because it's really cold in here and we have so many windows and cracks and draughts."
She said her two children - both under the age of five - normally have long bouts of bronchitis and asthma.
"When they do get sick, they get a lot of chest infections and sore throats and things like that because of all the mould."
She had asked her landlord about insulation and heating, but did not want to add to that for fear of eviction, she said.
"I am a bit worried about getting kicked out."
Mr LaPointe said the Government should legislate for mandatory curtains in rentals so tenants would not have to have those conversations.
But Housing and Urban Development Minister Phil Twyford said unless they were installed to "very precise" specifications, curtains "don't necessarily provide warmth or insulation".
"Our Government is committed to improving the quality of rental properties so our families are healthier and happier," he said.
"That's why one of the first things we did was to pass the Healthy Homes Guarantee Act to make rental homes warmer and drier."
But curtains were not "currently being considered under the proposed Healthy Homes standards," he said.
"The Ministry of Social Development can help low-income households buy curtains if they are not provided in their rented homes," Mr Twyford said.
"Tenants may also be able to find second hand curtains through community-run curtain banks."