It has been 15 months since Canberra's Roozi Araghi, his partner Luke Dorsett, Mr Dorsett's sister Kate Goodchild and Sydney mother Cindy Low died at at the Gold Coast amusement park on October 25, 2016.

All four died after an incident on Dreamworld's Thunder River Rapid ride, just after lunch on that day.

Mr Araghi's mother, Gay Araghi, spoke briefly from the family home in Sydney after Queensland's state coroners office said their team was still sifting through reports from police and Workplace, Health and Safety.

"We have absolutely no idea," she told Fairfax Media.

"We are still waiting on the information and results from the inquest investigation."

Kim Dorsett, the mother of Mr Dorsett and Ms Goodchild, said the family also did not know when the inquest would begin.

"All we know is that it is ongoing and we've been given no timeframe," she said.

"We all know exactly the same thing and we have been given no timeframe.

"We are kept informed every month. It takes as long as it takes. I've never been through this before, so I don't know how long it takes.

"That aside, all of us are anxious about the time it is taking. But it takes as long as it takes."

Efforts were made to contact Ms Low's family.

Police and Workplace Health and Safety teams have each investigated the Dreamworld tragedy and gave their reports to the Queensland's Office of the State Coroner in 2017.

The OSC said in a statement the inquest would start with a preliminary hearing in 2018 before the formal coroner's inquest.

"The matter will be opened with a pre-inquest conference - a preliminary hearing - but as previously advised, no date has been set down as yet for the hearing," an OSC spokesman said.

The spokesman said there was not a backlog of coronial inquiries halting progression towards a Dreamworld inquiry.

Last year, Queensland Attorney-General Yvette D'Ath ordered new coronial inquiries to both the March 1973 Whiskey Au Go Go nightclub firebombing in Fortitude Valley, where 15 people died, and the disappearance of Ipswich woman Sharron Phillips in 1986.

The OSC was also asked to investigate if there were grounds to for a new coroner's inquiry into former Rat Pack informant, sex worker Shirley Brifman, who was found dead at her home in 1972.

The OSC agreed the complex Dreamworld inquiry was taking time.

"While there is understandably great public interest in the investigation, it also needs to be understood that a significant number of highly complex reports, due to the nature of the tragedy, are and have been compiled as part of the coroner's investigation," the spokesman said.

"An inquest into the four deaths will proceed, however a date is yet to be determined."

South-east Queensland coroner Jim McDougall is investigating the four deaths on the Thunder River Rapids Dreamworld ride.

The spokesman declined to say if the police or the Workplace Health and Safety investigations recommended charges against Ardent Leisure, which owns and operates Dreamworld.

"As this matter is an open coronial investigation, no further information will be released at this time," the spokesman said.

Queensland state coroner Terry Ryan will run the Whiskey Au Go Go inquest and Queensland Police's Homicide Squad is leading the investigation.

"A number of people have contacted the Coroners Court offering to assist with the investigation," a spokesman said.

Anyone who believes they have relevant information to this inquiry were asked to contact the State Coroner by emailing state.coroner@justice.qld.gov.au or by contacting the Homicide Investigation Unit on 3364 6122.

A coronial inquiry into Ms Phillips' disappearance is still being investigated by homicide detectives.

Brisbane detectives in October 2017 named deceased Redbank Plains taxi driver Raymond Peter Mulvihill as the man who murdered Ms Phillips in 1986.