Granddaughter Shaquille Spooner-Nixon (22) said the police investigation had "come to a halt" and police officers had stopped responding to requests for updates on the case.
She said the police were not interested in new information the family was offering, including screenshots of people "boasting" on social media about the attack.
"Nothing we had was ever good enough for them to act on. We keep getting shoved to the side," Miss Spooner-Nixon said.
Mrs Spooner said the names were given to police but were never acted on because it was considered hearsay.
"Everything is hearsay until you investigate it and prove it otherwise."
Police had "swept the case under the carpet and put it in the too-hard box and buried it", Mrs Spooner said.
Otago Coastal Acting Area Commander Inspector Kelvin Lloyd said the investigation remained open.
"Police have thoroughly investigated lines of inquiry as they have opened up. There are a number of complex factors and dynamics associated with the case.
"Detectives have met with the victim on a number of occasions and have had open dialogue. The victim has been updated on the progress of the case on several occasions. Police welcome any new information which could help them with their inquiries."
After the Otago Daily Times contacted police with questions about the case, a police liaison officer called Mrs Spooner and made an appointment to talk to her tomorrow.
Mrs Spooner lodged a complaint with the Independent Police Conduct Authority about the police investigation.
The response from the authority was "rubbish".
A letter from the authority said: "unless more information or evidence is found, we can't proceed any further with this case".
"The case file will be kept by police at Southern File Management Centre and if more information or evidence is found we will contact you," the letter said.
The authority response made Mrs Spooner angry because she believed the police were provided with enough names to progress the investigation.
"I thought it was a cop-out, I really did."
Miss Spooner-Nixon said the complaint "backfired" on the family.
It gave police a reason to stop communicating with them on the case because it was under investigation.
Authority complaints manager Pieter Roozendaal said the authority did not discuss complaints and complainants.
"However, I can confirm that all complaints received by the authority are actioned in accordance with the provisions of the IPCA Act 1988. If Mrs Spooner had indeed complained to the authority, then she would also have received correspondence informing her exactly how her complaint was to be dealt with."