Australia helicopter collision: We need to know what happened in cockpits - officials
Investigators are still trying to work out what caused a deadly mid-air collision between two helicopters on Australia's Gold Coast, officials say.
A British couple and two Australians died in the crash on Monday near the Sea World resort in Queensland.
Three others - including two children - were seriously injured in the crash.
Officials say it happened less than 20 seconds after one helicopter took off from a sandbar and collided with another aircraft that was landing.
"What we do need to know now is what was occurring inside those two cockpits at the time," air safety commissioner Angus Mitchell said.
All those killed and critically injured were in the ascending helicopter, which crashed within seconds after its main rotor blade struck the cockpit of the other aircraft.
The second helicopter landed upright on the sandbank. Five of the six people on board suffered minor injuries.
The landing was a "remarkable achievement" given the helicopter was damaged "where the pilot was sitting", Mr Mitchell said.
"We are very fortunate that we're not standing here with far more deaths," he told reporters on Tuesday.
The UK victims were Diane and Ron Hughes from Merseyside - aged 57 and 65 - who were married and had next of kin in Australia.
Australian media reported that 36-year-old Vanessa Tadros also died in the crash, and her 10-year-old son was among the survivors hospitalised in a critical condition.
The fourth victim was 40-year-old Ashley Jenkinson, an experienced Sea World Helicopters pilot who lived in the area. The Brisbane Times reported he was originally from England.
A close friend of Mr Jenkinson's, Ritchie Gregg, told the BBC he was a "gentle giant" and a "silent hero".
"He was always out there helping the community - from the bushfires a few years ago to the flood waters last year," he said.
"What we did with the floods in NSW last year was definitely a highlight of my life. If it wasn't for him, a lot of people wouldn't have got the help that they got."
New South Wales was devastated by floods last year, particularly near Lismore.
A boy, 10, and a woman, 33, remained in a critical condition in hospital on Tuesday, police said. A nine-year-old boy with serious injuries was described as stable.
Both helicopters were operating tourist flights for Sea World - one of several popular theme parks on the Gold Coast. Its owner, Village Roadshow Theme Parks, has offered condolences and said Sea World Helicopters is an independent operator.
At the scene
Shaimaa Khalil, BBC News
It's difficult to square how beautiful and serene that spot of The Broadwater on the Gold Coast is with that fact it is now a major crash scene. The water is clear, there are boats and jet skis all around.
Just opposite is the sandbank where the helicopter crash happened. The wreckage of the choppers was removed earlier; authorities were finding it quite challenging before because of the tide. Now a couple of police boats remain with investigators scouring the area for more evidence. I've seen at least three divers going underwater to scan the sea bed.
A stone's throw away is the Sea World theme park where people can be heard on the rides. One eyewitness said that as one of the helicopters was trying to land it hovered over the park and there was concern it would crash into crowds. This is peak holiday season and thousands are here with their children.
"If you know the Gold Coast you'd know Sea World helicopters," one local told me. They are a common attraction and many tourists fly on them on quick tours around the area.
There are still many questions the investigators are trying to answer - most crucially is why one chopper took off in such close proximity with another that was trying to land.
The ATSB says it will conduct interviews and meticulously scour the helicopters, scene, footage, and other evidence before speculating on what caused the crash. It called on eyewitnesses to come forward.
Police said boat operators had rushed to help after the crash at about 14:00 local time (04:00 GMT), including by giving CPR.
Authorities had faced challenges accessing the sandbank and later securing evidence amid tidal changes, officials said.
The ATSB's preliminary report is due in six to eight weeks.
Prime Minister Anthony Albanese said the country had been left shocked by the "terrible and tragic incident".
The UK Foreign Office has said it is supporting the victims' families and will remain in contact with local authorities.