Dreamworld inquest: The fatal 20 seconds after Thunder River Rapids ride catastrophe

A forensic crash investigator has told the second day of the inquest into the Dreamworld tragedy there were 20 critical seconds after the Thunder River Rapids rafts collided and if the emergency stop button was pressed it may have prevented some injuries.

Senior Constable Steven Cornish told Queensland's Southport Coroner's Court the slower stop button, which takes eight seconds to bring the Thunder River Rapids Ride conveyor to a halt, was pressed 10 seconds after the incident.

Under cross examination he was asked if the emergency stop button was pressed would it have avoided the tragedy which killed Kate Goodchild, Luke Dorsett, Roozi Araghi and former Kawerau woman Cindy Low.

Dreamworld victims Kate Goodchild, Luke Dorsett, Roozi Araghi and Cindy Low.

"[It] would not have avoided it," he said.

"It may have limited some injuries. Possibly."

The first day of the inquest heard the ride operator close to the emergency stop button, which takes two seconds to work, did not know what the button was for and was told not to worry about it.

Cornish today told the court there were 10 seconds between the rafts first hitting and the slower emergency stop button being pressed.

Dreamworld operator Courtney Williams dived in to help rescue people from the tragedy.

The rafts first collided at 2.05.03pm on October 25, 2016 and the conveyor belt on the ride did not come to a complete stop until 2.05.22pm - almost 20 seconds after the first collision.

He said based on how long it took for the ride to slow, it appeared the slower emergency stop button was pressed.