Police say they haven't received any groundbreaking new information in the Israel Jack unsolved murder case since a $10,000 reward was offered.

The reward was offered by an anonymous member of the public on Tuesday last week but was not done in conjunction with the police.

Police have confirmed they will "reassess" their investigation from five years ago and look into new information passed to them by Israel's father, Rendall Jack.

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The father blames a gang for his son's brutal killing on Te Ngae Rd on August 18, 2013. Police also confirmed for the first time recently that that was "probably" the case.

Israel's body was found lying on Te Ngae Rd, between Pohutukawa Dr and Robinson Ave, at 4.25am after it had been run over by two vehicles not linked to his killing.

Police said his death was considered a homicide because less than five minutes earlier, at 4.20am, he was seen on security camera footage leaving Caltex Te Ngae, 1.4km from where his body was found. Police said Israel could not have walked that distance in that time.

Rendall Jack said he had information his son was set upon by a group of gang members after he left the Caltex Te Ngae. His body was then left lying across the road just minutes before he was unwittingly run over twice.

A man, who the Rotorua Daily Post agreed to keep anonymous, came forward wanting to offer the $10,000 reward, saying it was time Israel's killers were brought to justice.

He said he would set the money up in a trust account held with a lawyer and the money would be paid to a witness or witnesses after a murder conviction, and after all appeal processes had been exhausted.

The anonymous donor said he knew the names of the killers but didn't have the evidence.

"The witnesses are scared so that's why I'm making this offer."

The man said there were young women who he believed were key witnesses and hopefully they had "grown up" now and were prepared to do the right thing.

Detective Sergeant Tony Colby said the police had received one phone call since publicity about the reward but he wasn't yet sure if it would lead to anything.

He said police would talk again to some key people in the hope something new would come out.

He said it wasn't necessarily a long shot to hope someone might now come forward five years later.

"People contact police for a variety of reasons and we are happy to hear from them."