Specialist investigators are examining the wreckage of two helicopters that collided in mid-air on the Gold Coast to determine how the tragedy that killed four people unfolded.
The two aircraft, operated by Sea World Helicopters, were conducting joy flights over the Broadwater near the theme park on the New Year's Day public holiday when they crashed into each other.
Australian Transport Safety Bureau (ATSB) personnel are now sifting through the wreckage, analysing footage of the collision and interviewing witnesses and operators, to determine what led to the crash.
Here's what we know about the crash.
How did the crash happen?
What caused the two helicopters to collide mid-air is still to be determined but ATSB Chief Commissioner Angus Mitchell said the crash occurred below 150 metres in the air about 2pm on Monday.
He said one helicopter carrying seven people was taking off and the other helicopter with six people on board was landing at the time of the crash.
The main rotors of the helicopter taking off collided with the landing helicopter and caused "considerable damage" to its front section.
It caused the main rotor blades and gearbox to separate from the ascending aircraft, which then crashed down onto the sandbar.
Mr Mitchell said it was remarkable that the second helicopter had been able to land upright.
The ATSB says the take off and the landing are "critical phases of flight" where the "cognitive load of pilots is at their greatest".
What do we know about the victims?
The pilot of the helicopter that was taking off and three adult passengers were killed in the crash.
Sea World Helicopters pilot Ashley Jenkinson, New South Wales woman Vanessa Tadros and UK couple Diane and Ron Hughes were killed in the mid-air collision on Monday afternoon.
Three passengers remain in hospital: Geelong woman Winnie De Silva, 33, and her son, Leon, 9, were critically injured, along with Ms Tadros's son Nicholas, 10.
Queensland Police Acting Inspector Mike Campbell said one of the deceased people was related to one of the survivors.
Three more people were critically injured, which police said include a mother and son and another child.
Thirty-three-year-old Geelong woman Winnie De Silva and her nine-year-old son were critically injured, along with a 10-year-old boy from New South Wales. All three remain in hospital.
How will authorities determine what happened?
Mr Mitchell said the ATSB is gathering all the intelligence and evidence it can, including vision from the helicopters, passenger footage and witness accounts.
He thanked community members who came forward with camera footage and CCTV from buildings so investigators could piece together what occurred.
"What we do need to know now is what was occurring inside the two cockpits at the time," Mr Mitchell said.
"We do have multiple accounts from witnesses and as I said, of video, but equally we'll get what we can from those two helicopters to try and get a better picture of what was occurring but also what were the processes in place that are designed to actually protect helicopters in this situation.
"We know it's a busy time of year, we know helicopters are in and out of that Sea World helipad constantly throughout the day so we'll certainly be looking at multiple lines of inquiry here around the procedures that were in place and potentially what has helped contribute to such a tragic outcome today."
The helicopters have been removed from the sandbar and taken ashore where ATSB investigators will get a better appreciation about what's occurred and the forces that are in play, Mr Mitchell said.
Anyone who witnessed the collision has been asked to come forward.
How did bystanders react?
Witnesses to the confronting scenes on the Broadwater were among the first responders to the scene, rushing to the aid of the helicopter passengers.
Acting Inspector Campbell said people were using their private boats to take people across to the island to reach the victims to render first aid.
Witness John Hunt told the ABC he feared one of the helicopters would crash on top of the crowd at Sea World.
Another witness Amii McDermott said she could hear the engine of one of the helicopters starting to rev "like the pilot was taking some sort of emergency action".
What support is in place?
Acting Inspector Campbell said there are family support measures in place.
Village Roadshow Theme Parks, which runs Sea World, said they were providing support to "team members who have been emotionally affected by the tragedy".
