In a statement they said that an RP16 or R16 rating was "discussed" but dismissed as not being strong enough.
"It was decided that would not address the harm caused to sixteen and seventeen year olds who are at statistically greater risk of suicide."
The drama, which tackles the topics of teen suicide and rape, frequently goes into graphic detail and has, unsurprisingly, caused much discussion.
While some have praised the series for shining a light on these dark, often unspoken topics, others have expressed concern that its confronting nature was inappropiate for the age group it was attempting to reach.
The Mental Health Foundation expressed dismay that 13 Reasons Why was even released, saying they belived it was putting young people at risk.
Speaking to Radio New Zealand's Morning Report this morning the Deputy Chief Censor Jared Mullen explained the reasoning behind the new rating.
"We know that kids know they're not supposed to watch something and still do," he said. "The key thing is that teens and parents have the information to make a decision, that's really what the classification is about."
Mullen did not rule out using the rating again in the future.
"If this show is any indication, we may well need to use this classification again.
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• 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)
• Rainbow Youth: (09) 376 4155
• Samaritans: 0800 726 666
• If it is an emergency and you feel like you or someone else is at risk, call 111.