Three other people were in critical condition, police said, describing how bystanders scrambled to pull passengers out of an overturned helicopter.
A helicopter capsized on the exposed sandbar a few meters from the shore, according to images from the public channel ABC.
The aircraft had its rotors removed, which were located a short distance away.
The other helicopter appeared to be largely intact at the scene of the crash, which is near the Gold Coast’s popular Sea World marine theme park.
A bright yellow rescue helicopter had landed on the nearby arena, its rotors spinning, aftermath footage showed, with dozens of rescuers fanning out around the area.
Several small police and rescue boats were moored on the exposed sandbar, which was littered with pieces of the wrecked helicopter.
“Those two planes, when they collided, crashed and landed on the sandbar just outside the Sea World complex,” Queensland Police Service Acting Inspector Gary Worrell said at a news conference at the site.
“As a result of that, four people have lost their lives today,” he said. “Three others are critical in the hospital.”
– Investigation started –
One of the helicopters appeared to have taken off while the other was landing, Worrell said, adding that plane crash investigators had yet to determine the facts.
The helicopter that landed safely had lost its windshield, the police office said. “The other fuselage crashed and was upside down.”
“Members of the public and police attempted to pull people out and began first aid to try to get those people to safety (from) the upside-down fuselage.”
Access to the scene was complicated by the location of the sandbar, Worrell said.
“I would like to thank everyone who came out and made it possible to get those people to emergency care like you did.”
Australia’s transport safety office said it had already launched an investigation into the mid-air collision.
Investigators would examine the remains and map the site, as well as recover the components for examination, it said in a statement.
The office appealed to anyone who saw the helicopters in flight or had any footage to get in touch.
A preliminary report was expected in six to eight weeks.