The Glen Innes attack happened a day after Auckland man Tom Coombes was fatally attacked in a Mt Albert walkway. A 22-year-old man has been charged with his murder.
In an emotional Facebook post, the victim's sister has appealed for information and shared disturbing details of the incident.
She claimed that "three teenage males" initially attacked her sister's boyfriend but then turned savage towards his partner when she tried to defend him.
As a result of the attack the 20-year-old sister had now lost vision in her left eye and it was uncertain whether she would ever be able to see again.
"She has to have her head down 24/7 now otherwise her eye will pop before it gets a chance to heal... according to the ophthalmologist," said the distraught sister.
She said she was in desperate need for help and asked for people who knew anything about the attack to come forward.
Police asked anyone with information to contact 105 and quote the file number 220525/2291. Alternatively, information could be given anonymously by calling Crime Stoppers on 0800 555 111.