Four people have died after two helicopters collided mid-air near Sea World on the Gold Coast.
Three more people are in a critical condition in hospital, Queensland Police Service Acting Inspector Gary Worrell said.
All of those who died or were seriously injured were from the same helicopter, he said.
He said the aircraft landed on a sandbank outside the Sea World resort, making it difficult for emergency crews to access the crash scene.
Police and witnesses said one helicopter was landing and the other was taking off when they collided.
One helicopter plunged to the ground face down and the other managed to successfully land — a Sea World logo could be seen on its side.
"One air frame has the windscreen removed and it's landed safely on the island, the other airframe has crashed, it was upside down," Acting Inspector Worrell said.
The Queensland Ambulance Service (QAS) said the incident occurred at 1:59pm and 13 patients were assessed at the scene.
Eight patients were taken to Gold Coast University Hospital and one to Queensland Children's Hospital in Brisbane, QAS said.
It comes at the height of tourism season at the popular holiday destination. Sea World Drive at Main Beach has been closed, with motorists and pedestrians advised to avoid the area.
Australian Transport and Safety Bureau (ATSB) is investigating the collision and appealed for witnesses to come forward.
"The ATSB has commenced a transport safety investigation into the fatal mid-air collision involving two helicopters on Monday afternoon
"Transport safety investigators with experience in helicopter operations, maintenance and survivability engineering are deploying from the ATSB's Brisbane and Canberra offices and are expected to begin arriving at the accident site from Monday afternoon," ATSB chief commissioner Angus Mitchell said in a statement.
"During the evidence gathering phase of the investigation, ATSB investigators will examine the wreckage and map the accident site."
A spokesperson for Sea World Helicopters confirmed both aircraft belonged to the company.
"We and the entire flying community are devastated by what has happened and our sincere condolences go to all those involved and especially the loved ones and family of the deceased.
"We are cooperating with all the authorities including the Australian Transport Safety Bureau (ATSB) and the Queensland Police. As it is now a police investigation we cannot provide any further information at this stage."
Incident unfolded 'in seconds'
Amii McDermott witnessed the crash while she was standing in line for a ride with her family at the Sea World theme park.
She said it seemed to have occurred when a helicopter coming into land collided with another that had just taken off.
"We were looking to where they were taking off and we saw two helicopters and they were close, which was unusual because normally you didn't see them together that close,'' she said.
"Then I heard one of the engines in one of the helicopters start to rev like the pilot was taking some sort of emergency action."
Ms McDermott said the tail of the helicopter coming into land hit the front of the helicopter taking off.
"It started smashing into it … and then it continued to hit it [the cabin of the other helicopter] and then the tail pretty much snapped off,'' said Ms McDermott who was visiting from the Blue Mountains.
Ms McDermott said after the tail fell off, the helicopter then just dropped.
"It plummeted to the ground nose first, straight down."
She said the other helicopter was wobbling but went down trying to land.
"It didn't look good but it was maintaining some control."
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Ms McDermott said no-one was screaming because nobody knew what was happening and it occurred so quickly.
"Unless you happened to be looking in the right direction you would have missed it. It was over in seconds,'' she said.
As she left with her family she saw first aiders running into the theme park entrance.
"They were calling out: 'Bring the defibrillators','' she said.
Windy day in the area, witness says
Nearby resident Steve Raschilla said he saw one of the helicopters "break into pieces".
He said he heard a "loud bang" and pieces of the helicopters began to fly everywhere.
"The main part of the helicopter was intact and fell straight to the ground and lots of other bits and pieces of bits in the middle, they all just fell to the ground," Mr Raschilla said.
"And then the other helicopter shortly after landed."
Mr Raschilla said it was a usual spot for helicopters to fly, but it was odd seeing helicopter out in windy conditions.
"What seemed strange today was that it was so windy, and helicopters were out, and so close together," he said.
Hannah, who was at Sea World at the time of the incident, said she initially thought it was a massive car crash.
"It sounded like a car crash to begin with, a big one, and then we heard the helicopter propellers slowly getting lower, and then we heard another massive big bang," she said.
"A couple [of] people kind of stopped and looked around and was like, 'what's that' and everyone else just went to went along with their business."
The crash comes almost 32 years after a joy flight from Sea World ended in disaster when a helicopter crashed into the beach on South Stradbroke Island, exploding into flames, killing all seven on board.
In that case on March 3, 1991, five members of the one family died in the crash, along with a family friend from France, and the 27-year-old pilot.
At the time, the 1991 crash was described as the worst helicopter crash in Australian history.
Five years later, two Black Hawk helicopters collided mid-air during a counter-terrorism training exercise in Townsville, killing 18 men in Australia's worst peacetime military aviation disaster.
