Police have been using blood samples taken from people at birth to solve crimes and identify bodies.
Since 2006, police have been able to use blood "spot samples" taken from people at birth to get DNA samples under an agreement with the Ministry of Health.
The samples have been taken from newborns, with their parents' consent, since 1969 to screen for diseases and the remains are stored unless parents request their return.
Police have now confirmed that in 2010 they used a spot sample to identify human remains, along with two successful homicide prosecutions - in 2010 and 2012 - although those also required court-ordered warrants.
They were also used to identify 13 victims of the Christchurch earthquake.
A police spokeswoman said they were used "rarely and as a last resort".
"Information shared ... is tightly protected and bound by all relevant laws, including the Privacy Act, the Official Information Act ... and the Human Tissue Act," she said.
The agreement with the ministry did not override any of those laws, she said.
Comment has been requested from the Privacy Commissioner and the Ministry of Health.
The National Screening Units website explicitly states that left over blood samples can be used for forensic work by police.
To date, they have not been used for large-scale population studies, it said.
But Judith Furlong, the mother of killed teenager Jane Furlong, told Fairfax she was stunned when she found out a sample from her daughter had been used for identification.
"Forty-two years ago they certainly didn't say they were keeping it. I had no idea," she said.
"It's invasive because they didn't inform you of anything."