NSW Police have refused to explain why it took eight months to reveal to the public the disappearance of a 15-year-old girl who had vanished while in State Government care.

Key points: Katrina Bohnenkamp grew up in foster homes or in group homes

Katrina Bohnenkamp grew up in foster homes or in group homes A coronial inquest into her disappearance is set to begin next week

A coronial inquest into her disappearance is set to begin next week Police said they had "grave concerns" for her welfare

Officers on Tuesday announced a $500,000 reward for information leading to the conviction of persons responsible for the disappearance of Katrina Bohnenkamp.

The Sydney girl was last seen in 2012 at a home in Strathfield, according to police, who have now revised the date of her last confirmed sighting by a week.

"It's like they're finally taking it seriously," Lee Schellnegger, Katrina's cousin and former carer, said of the police reward on Tuesday.

"Might be a bit late though."

Ms Schellnegger said police should have made more appeals earlier in the investigation when people were more likely to remember information.

A coronial inquest into Katrina's disappearance is set to begin next week, but the homicide squad have not taken charge of the case during the seven years she has been missing.

Katrina grew up living in foster homes, with relatives, or in group homes — the NSW Minister for Community Services was legally responsible for her welfare during this time.

The teen often ran away from carers and had left a residential group home in Sydney's west without permission before she was reported missing.

A spokesperson for the NSW Department of Communities and Justice, formerly Community Services, said it no longer counted children as being in state care if they were "absent location unknown".

They said the new rules ensured greater accuracy and were in line with national guidelines.

Katrina's family has been devastated since her disappearance. ( Supplied: NSW Police )

Katrina's sister Jaide Simpson said she had spent the past seven years in "emotional limbo".

"It's been horrible," she said, speaking at police headquarters at Parramatta.

"Not having any answers, not seeing her, not knowing what happened to her, and for that to continue year after year after year, with still nothing, has been heartbreaking."

Jaide said her sister was fun, hyperactive, kind and "just wanted to have the best life".

NSW Police did not say why there was a delay before putting out a media release to say she had gone missing.

The officer in charge of the investigation, Detective Inspector Christie Houldin, said that was a matter for the coroner.

"We obviously hold grave concerns for Katrina's safety," she said.

Ms Houldin would not say why the case was still being handled by local police at Auburn and not the homicide squad.

She also declined to discuss police theories as to what happened or name persons of interest.