"I address this to the 'haters' that took it upon themselves to try and dim Mexha's light. You have successfully snuffed out her light and robbed the whole world of her and in turn, shattered mine," Sane writes.
"My plea to you and to everyone - change your ways."
But Sane also offered the bullies forgiveness and said she wished them no harm.
"Respect each other, be kind, be mindful," she said in the letter, which she posted on Facebook.
"Strive to be a better person. I wish no harm on you, and I'm thinking of your mothers and your families when I say this."
Sane told the Herald on Sunday she hoped sharing her story would help bullies realise how dangerous their words could be.
Mexha was not a likely target. She was a New Zealand age-group karate champion and was training hard for the Junior World Championships in Jakarta next month. She worked hard, was respected by teachers at Onehunga High School and was popular with her karate instructors.
But bullying started in Year 10 and was dealt with by the school.
Sane didn't realise the bullying had started again until it was too late.
"I didn't know she was being bullied again but a few of the kids have said she was," Sane said.
"It centred around a boy and jealously, just girls being mean."
The diary found in Mexha's room confirmed the truth.
Sane said she will never read the diary but will give it to police who are investigating Mexha's death.
She said police had taken Mexha's cellphone and were checking her last emails and social media posts. Police were unable to comment yesterday.
Martin Cocker, executive director of NetSafe, said cyber-bullying was prevalent among young people and easily hidden from parents.
"These things can escalate very quickly and children can find themselves in a situation they feel they have no control of."
The new Harmful Digital Communications Act, which passed into law in June, makes it illegal for people to post digital communications that have the intention to cause harm to someone else.
Part of the legislation includes the creation of an agency specifically to support young people experiencing cyber-harassment, Cocker said.
"I can't say it would have saved this young person, but it will help."
Onehunga High School principal Deidre Shea told the Herald on Sunday she could not comment on Mexha's death, but the school was offering support to the girl's family, friends and fellow students. "We are working to support everyone through this difficult time," she said.
Shea said the high school was committed to a positive environment. Programmes were in place to address bullying.
"Unfortunately bullying is a universal issue for schools, workplaces and communities," she said.
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youth services: (06) 3555 906 (Palmerston North and Levin)
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
• Samaritans: 0800 726 666 (available 24/7)
If it is an emergency and you feel like you or someone else is at risk, call 111.
