Premier Annastacia Palaszczuk said the event that left 12 people dead and caused 50 families to lose everything still had question marks over it.

She highlighted a DHI consultant report, published by The Australian in March, which suggested "several inconsistencies" as one of the reasons for the inquiry.

"The report has found that a section of embankment that collapsed at the Grantham quarry was 260 metres long, not 55 metres as initially reported by the Queensland Floods Commission of Inquiry," Ms Palaszczuk told parliament.

"The DHI report has found there were also breaches of the quarry wall rather than just a single breach.

"Grantham residents have come forward supporting the findings of the DHI report. The people of Grantham want the truth and that's why my government is launching this Commission of Inquiry."

Chaired by well known QC and former Solicitor-General Walter Sofronoff, the inquiry will begin gathering evidence next week and is to be completed by August 31.

"It is proposed that the consultation phase of the Inquiry will be limited to about two weeks, but I stress this Inquiry will be independent," Ms Palaszczuk said.

The inquiry will focus on:

• The impacts of man-made and natural features of the landscape which could have altered or contributed to the flooding
• Whether the existence or breach of the Grantham quarry caused or contributed to the flooding
• Whether the quarry had a material impact on the damage caused
• Whether the breach of the quarry had implications for the evacuation of the town
• ​How these matters were initially investigated and how eyewitness accounts were dealt with in the aftermath.

"The people of Grantham have suffered through an horrific, terrifying, fatal event," Ms Palaszczuk said.

"They deserve and require further closure. They want the truth to prevail."