Inland Revenue is sitting on more than $200 million of charitable Kiwis' money, with almost half not getting their donation rebates refunded.
Any New Zealander who donates more than $5 to a registered charity can claim 33.33 percent of it back from IRD if they have a taxable income and have a receipt.
A law change in 2014 put a four-year deadline on claims.
The latest figures from IRD for the 2015 tax year show the department processed a rebate from just under 370,000 people - around 54 percent of those who could apply. The figure is based on donations recorded by the Charities Service.
Tax Management NZ claims the number of Kiwis not getting their rightful money back has fallen 10 percent since 2008.
But IRD says rebate levels have been "roughly" 54 percent over the past few years with a few fluctuations.
The company estimates there's more than $200 million in unclaimed rebates.
IRD says any money not claimed is "effectively revenue for the Government", just like any other money owed which isn't claimed.
It has partnered with a number of charities including KidsCan, The Mental Health Foundation, Forest & Bird, Tearfund and Starship Foundation to give back and help process those claims on behalf of those who've donated money to worthy causes.
Tax Management NZ says the work isn't their main business, but thinks the community should be rewarded for their generosity which sees about $1.3 billion donated to the 27,816 registered charities each year.
"We hope that by using our place within the New Zealand tax system we can help more New Zealanders who give to charity to claim from IRD the rebate they're entitled to and, should they wish, give even more to the causes they are passionate about."
An IRD spokesperson says there could be a number of reasons people don't make a claim, which include the donated amount being so small it isn't worth the relatively little effort.
"People also need to keep the receipts for the donation which may again be relatively onerous for small claims. Some people also consider making a tax rebate claim to be contrary to the spirit of making a donation to a charity," the spokesperson told Newshub.