There has been a breakthrough in one of Canberra's longest-running missing persons cases following the death of the prime suspect.

Police said a number of people had given them fresh information about the disappearance of Megan Mulquiney, since serial sex offender Paul Vincent Phillips died in April.

Ms Mulquiney, then 17, was last seen outside the Woden Plaza on Saturday July 28, 1984.

She had completed her shift at Big W and was due to catch a bus home, but never arrived.

Almost 34 years later to the day, ACT police have announced they do not believe Phillips acted alone.

A 2009 inquest into Ms Mulquiney's disappearance determined it was extremely likely she was murdered by a person or persons.

Since Phillips' death in April, Senior Constable Patrick O'Brien said a number of people had come forward to assist police with fresh information.

"We believe there are people in the community who do know the circumstances of Megan's disappearance, or may be able to assist us in our investigation," he said.

"It may be that those people carry a significant burden, whether they know the whereabouts of Megan or the circumstances — we implore them to come forward and speak with us."

Megan fit profile of Phillips' victims

Phillips spent the majority of his adult life behind bars for a series of violent sexual offences against young women.

Phillips was questioned during the 2009 inquest into Megan Mulquiney's disappearance. ( ABC News )

Detective Senior Constable Emma Beere said he lived a nomadic lifestyle and, while in Canberra, stayed in 17 different locations, including caravan parks and camping grounds.

"Paul Vincent Phillips was known to prey on his victims as they entered open carparks," she said.

"One such incident occurred only two months after Megan was last seen leaving her workplace in Woden Plaza.

"His victims would often appear young, have child-like features, they were petite in build … and would wear their shoulder-length hair out.

"This description matches that of Megan."

Ms Mulquiney was last seen at Woden Plaza after finishing her shift at Big W. ( Supplied: ACT Policing )

During the 2009 inquest, Ms Mulquiney's mother Dorothy said she believed Phillips had some involvement in her daughter's disappearance.

"I know [the DPP] tried really hard to get an admission out of him," she said.

Ms Mulquiney said she did not hold out hope that Megan was alive.

"But I'd like something to be found out so I can put it to rest, because without knowing it's a nightmare," she said.

"As a mum, I really need to know what happened to Megan."

Anyone with information has been urged to contact ACT Policing on the dedicated mobile number 0457 844 917 or speak to them in person at Woden shopping centre this Saturday at midday.

Information can also be provided anonymously via Crime Stoppers on 1800 333 000.