The boyfriend of murdered teenager Jane Furlong has refused to speak to police since her body was found, but the officer in charge of the investigation hopes a $50,000 reward will "loosen" others who hold important information.

Ms Furlong's mother has criticised those staying silent, calling them "self-preserving cowards".

The 17-year-old mother and part-time sex worker disappeared from Karangahape Rd in May 1993. Her boyfriend, Danny Norsworthy, reported her missing two days later.

Her remains were found at Sunset Beach, Port Waikato, in May last year.

Despite a major police investigation spanning almost 20 years, an arrest is yet to be made.

Yesterday, police announced a $50,000 reward for anyone who provided information or evidence that would lead to the conviction of Ms Furlong's killer.

Detective Inspector Mark Benefield believed more than one person was involved in the murder, or at least held vital information.

Police said they had spoken to hundreds of people and had identified "significant people of interest".

Ms Furlong's mother, Judith, said she was disappointed Mr Norsworthy would not speak to police.

"It stands to reason that he must know something - it's a logical deduction. I just wish he would talk.

"For all these years he's kept telling me over and over how much he loved Jane. Well, if that's the case, he better say what he knows."

Mrs Furlong said that on Monday police visited her Auckland home and asked her about Mr Norsworthy.

Mr Benefield said people involved in court cases Ms Furlong had been due to testify in for the prosecution were also refusing to speak to police. The cases involved gang members accused of an assault and businessman Stephen Collie, charged with brutal attacks on sex workers.

Mr Collie yesterday said police had not contacted him. "I've got nothing to say to them. I was in remand prison when Jane Furlong went missing."