"I just want to stop or try to help stop what happened to Emily happening to another girl," he told the Herald.
"The launch is on the anniversary on purpose; it was planned. It's difficult for me, but it's a good day to remind people what happened and why we need organisations like White Ribbon."
Emily's family lived in Auckland but she was attending school in Britain when she met Turner.
Turner admitted he had threatened to kill Emily up to 15 times and had even asked his friend to kill the teenager using a hammer.
The night before he strangled Emily, Turner hit her head against a table in a bar.
The incident was seen by several of the couple's friends.
Mr Longley believes Emily could still be alive if someone had challenged Turner.
The White Ribbon Campaign encourages men to not stay silent about violence towards women.
"The message for men not to condone, tolerate or put up with violence against women really resonated with me," Mr Longley said.
"Elliot Turner was so vocal and open about what he was going to do to Emily before he killed her but none of his friends stepped in or tried to stop him or warn her.
"When I found out about that I was shocked. If people had stepped in Emily could still be alive today."
He said that three years on, Emily's death remained hard to accept.
Mr Longley joins two other White Ribbon ambassadors - national RSPCA chief executive Ric Odom and District and Family Court judge Peter Boshier.
On the web
www.whiteribbon.org.nz
www.facebook.com/whiteribbonnz
Getting help
• Family violence information line 0800 456 450
• Women's Refuge crisis line 0800 733 843
• Stopping Violence Services website: nnsvs.org.nz
• If you or someone else is in danger, call the police immediately on 111
