A 12-year-old girl from Gisborne was bullied about her heart condition on an anonymous app, and told to "die now", leaving her parents "disgusted".
The anonymous message on the app Sarahah said: "Haha you have a hole in your heart...maybe you should die now cause your gonna die from it anyway lol."
Her parents, who did not want their child to be identified, saw the message during a regular check of the girl's phone.
"We just couldn't believe it. What a thing to say to a person who has been to hell and back," her mother told Newshub.
The girl has a serious heart condition, which required her to have three triple bypass surgeries before she turned four.
"You can only imagine what we've been through as a family," her mother said.
The girl's parents bought her a phone so they could keep in contact with her about her heart condition, but they have now taken it away.
After seeing the message on Sarahah, they told their daughter the app was unsafe and inappropriate - even though many of her friends are on it.
Sarahah is run by an Israel-based company and is used by over 1 million people. It markets itself as a service "to help people self-develop by receiving constructive anonymous feedback".
Users can send and receive anonymous messages, but they are not able to reply to them.
The girl's mother told Newshub parents should be wary: "People think it's harmless, constructive criticism. But anyone can get on it and say what they want."
The girl is still allowed to go online on a computer at home where her parents can keep an eye on her, but they plan to replace her phone with a model that can't access the internet.
"We do what we can to protect her," her mother said.
Several online petitions have been started by parents who want Sarahah to be banned, including one with over 160,000 signatures.
A Netsafe spokesperson told Newshub parents and young people should think carefully before using anonymous apps such as Sarahah.
"Parents need to consider the risks involved due to the anonymous nature of the services - for example, bullying, abuse and unwanted contact," they said.
"Some of these services have safety tools that allow people to block others or report content, but some do not."
Netsafe has limited power when it comes to acting against apps such as Sarahah, but it can work with online content hosts to resolve complaints of harmful digital communications.
"One of the options available to the District Court under the Harmful Digital Communications Act is to ask online content hosts to release the identity of the person behind an anonymous communication," the spokesperson said.
If something happens online and you need help, Netsafe recommends you talk to a friend or a trusted adult, or for free advice call 0508 NETSAFE or email email@example.com.