A woman whose brother allegedly stabbed an 11-year-old boy in the head at a private school in Auckland says the "tragic incident" could have been avoided.

Ambulance at the scene of school stabbing Source: 1 NEWS

The 23-year-old says before the stabbing she told Pacific Christian School in Mangere her brother was "constantly" being bullied.

She has sent a complaint to the Ministry of Education about the school's response, saying that the "tragic incident that happened at the school ... could have been avoided", the NZ Herald reported.

The woman said "if my concerns about my brother were taken into consideration, this whole incident could have been prevented".

A spokesperson for the Ministry, Katrina Casey, told the Herald that any complaint would be investigated.

"If we do receive a complaint that involves either of the two children involved in the incident, we will pass this information to police," she said.

The 11-year-old boy who was stabbed remains in a critical but stable condition at Starship Hospital, but family say there are signs he is responding to treatment.

The boy's parents and family have been at his hospital bedside since the incident on Tuesday.

The boy alleged to be responsible for the stabbing has been removed from his family home and is now in the care of Child, Youth and Family.