Investigators didn't consider the impact a controversial quarry had on the deadly 2011 Grantham flood because they were told not to, an inquiry has heard.

Detective Inspector David Isherwood told the Grantham Floods Commission of Inquiry on Tuesday that his taskforce, set up to help state coroner Michael Barnes, initially looked at the overall reasons why 12 people died when flooding hit the small Queensland town on January 10, 2011.

The devastating impact of the Grantham flood in January 2011. Credit:Nine News

But Inspector Isherwood said that changed when an earlier probe, the Queensland Floods Commission of Inquiry, released its terms of reference on January 17.

That inquiry was to look at overall causes of flooding across south-east Queensland at the time, prompting Mr Barnes to narrow the scope of his coronial inquests to only cover direct causes of death.