An online alert system for missing children has been launched today, a project between the New Zealand police and Facebook.

Photo: Supplied

The Amber alert system - a project between the New Zealand police missing persons unit and Facebook launched today - works to notify the public through various channels when a young person is missing or at risk of harm.

Robyn Jensen, mother of Kirsa Jensen who went missing age 14, in 1983, was at the announcement.

The US alert system is an emergency broadcast network that uses online, radio, and television to notify the public about missing children. The system is an acronym for America's Missing Broadcast Emergency Response and named after Amber Hagerman, a 9-year-old abducted and murdered in Texas in 1996.

Her killer was never found.

Versions of the system are active in more than a dozen countries.

New Zealand Police Commissioner Mike Bush said the system was a valuable new tool.

"There have only been a very small number of abductions involving children in New Zealand's history, but other situations, such as where a young child goes missing from home and is at serious risk of harm, occur more regularly.

"When these sorts of incidents do happen, police take them very seriously and will consider every option available to us to locate a child we have extreme concerns for.

"Having the Amber Alerts system means we now have another useful tool to quickly contact the public in emergency situations.

"If we can use it to help save even just one child, then it is a system worth having," he said.

Police will activate an alert if it is believed a child or young person who is missing is at serious risk of harm and public assistance could help to locate them.

Once the alert is activated, people who are part of the Facebook community in the targeted search area will receive a notification at the top of their 'News Feed'.

People can then choose to share the alert with their Facebook friends to help spread the word.

The alerts include a photograph of the child, any important information about the circumstances in which they went missing, and an indication that there is an active search.

Facebook director of trust and safety Emily Vacher said Robyn Jensen was a symbol of hope and perseverance. She said the system in the US had safely found almost 900 children.

"We keep you, your family, and Kirsa in our thoughts.

"As of today New Zealand becomes the 14th country in the world to join the Facebook AMBER alert family.

"Today with the changes in technology this life-saving information is delivered to the public in just a matter of minutes and right to the palms of their hands.

"Today that village of people protecting children in New Zealand has been strengthened.

"It's very important to note that these alerts will occur very rarely. If one is seen on Facebook it means you may actually be in a position to help reunite a missing child with his or her family."

Police Assistant Commissioner Investigations Richard Chambers said New Zealand has 2.9 million active Facebook users.

The system means immediate alerts would be sent to anyone registered on Facebook within a 160km radius if someone under 18 was missing and there was immediate risk to that person

"Our mission is to be the safest country and this partnership will help us get there.

"Child abductions in New Zealand are rare, with eight missing in circumstances that appear suspicious since 1932, but that's eight too many," he said.

Ms Jensen has told RNZ's podcast The Lost it had become harder to cope with her loss as the years had passed and she was still desperate to find her daughter.