"In our view, this offender represented a very high risk of imminent violent offending," he said.
The placement near the Mangere East school was only temporary while another property was prepared, and there was security to contain the man.
However, Mrs Stirling said it was "unacceptable" and the school should have been told he was there.
"Corrections didn't even notify me. I found out through a member of the public. My issue is that in evaluating his risk to a community, how can they possibly think that him being this close to a school is a low level?"
Corrections told her it was unable to move the man, who has temporary name suppression, until today.
So on Tuesday, Mrs Stirling sent letters to parents of the 2000 students.
"I now have been left with no other option but to officially inform you so that you can take extra precautions to ensure your children's safety," the letter said.
Mrs Stirling said the man had allegedly been communicating with students from his property, and they had filmed his behaviour.
"One of the students was standing out there and he started making hand gestures to her. Shouldn't he not be allowed to make contact with anyone under 16?"
Mr Riach said the man was strictly supervised, with 21 special conditions attached to his release, including one year intensive monitoring and a further six years under an extended supervision order.
"The 21 conditions are extensive, and some of the most stringent imposed on an offender residing in the community. His compliance with these conditions will be closely managed by a team of experienced probation staff.
"At all times the offender has had two staff, employed by a contracted service provider, monitoring him.
"A security guard has also been present at the address. He is not able to leave the residence without the permission of a probation officer," Mr Riach said.
Neighbours of the new property were currently being notified, including other schools, he said. "No address would be approved if we considered that it presented an unmanageable risk to the safety of the community."
Suspected breaches should be reported to police, he said.
Police referred Herald questions to Corrections.