Parents are being urged to be vigilant after text messages encouraging a lethal choking game were sent to two South Auckland girls, prompting one to accidently kill herself.

The 12-year-old girl, who is believed to have attended a Papakura intermediate school, died last Friday.

Inspector Gary Hill, of Counties Manukau police, yesterday said another 12-year-old girl was involved in a "self-harming incident" the same day.

Both girls, who attended different schools, are believed to have received text messages that detailed how the "choking game" - where a person attempts to self-strangulate to receive a brief high - was done.

Six primary and intermediate schools in Papakura are affected.

In response, some have held assemblies to address the issue, others have sent warning newsletters and started compiling names of at-risk students.

A trauma unit from the Ministry of Education is assisting schools, police and Child Youth and Family.

The ministry did not respond to Herald inquiries yesterday.

At a press conference yesterday, Mr Hill said three youngsters had died in the past two weeks in self-harm incidents.

Only one was a result of circulating messages encouraging youngsters to self harm. Her death was a result of what police yesterday called "communication that encourage self-harm" - through written and verbal outlets including text messaging.

The other deaths - a 12-year-old girl and a 10-year-old boy - were not related to circulating messages.

Asked whether parents of the girl who died had noticed any worrying signs or were aware of any communication that their daughter might have received, Mr Hill said they had.

"Her parents were aware of the notice that had been brought to their attention that the school had formulated earlier in the day."

None of the children were known to each other and did not go to the same schools, police said.

The circulating messages seemed to be confined to the Papakura area.

"There's a lot of speculation going on out there. We certainly know of a number of schools within the Papakura area that communication has been flowing - verbally, written or through text messaging.

"We've already held meetings with schools. School guidance councillors are getting involved ...

And also the schools are running risk registers of some of the students in the school who need support around that."