Crews then located a second aircraft, a Jabiru carrying one man who was pronounced dead at the scene, one kilometre north of the first aircraft.

“Early stages would appear to be a midair collision,” Supt Calman told reporters on Saturday.

“Damage at the scene would indicate the first aircraft has come straight down and we do have witnesses that have reported seeing debris coming from the sky.”

Ambulance and fire crews are at the crash site and two crime scenes have been set up.

The planes were flying in uncontrolled airspace at the time of the accident.

Police believe the Jabiru plane had taken off from the nearby Oaks Airfield.

“The scene that we’re closest to here that has been impacted by fire, it would not have been a survivable impact,” Supt Calman said.

“The second scene that’s a kilometre north of here, that aircraft did not burn, but it would also not have been survivable.”

NSW Ambulance Inspector Joseph Ibrahim said 10 ambulance resources including a helicopter were deployed to the “confronting” scene but “nothing could be done”.

“The nature of the injuries were unsurvivable,” he said.

The sites, covered by bushland, are near two flying schools – Sydney Recreational Flying Club and Dave’s Flying School.

The area is popular with trainee pilots and recreational flyers, local councillor Suzy Brandstater said.

“Recreational flying is very popular and this is probably one of the closest places to Sydney that you can do it,” she told AAP.

The Wollondilly Shire councillor said the accident was a “terrible shock”.

“It’s horrific and I really also feel for community members who saw it,” she said.

“It’s something you’re never going to forget – seeing two planes crash.”

Authorities are urging the public to stay away as forensics and police rescue vehicles move around the area.

The Australian Transport Safety Bureau will deploy a team to the crash sites to interview witnesses and collect air traffic control and flight tracking data, pilot and aircraft maintenance records, and weather information.

“Over coming days, investigators will undertake site mapping, examine the wreckage of both aircraft and recover any relevant components for further examination at the ATSB’s technical facilities in Canberra,” the safety regulator said.

They have urged any witnesses or anyone with footage of the planes in any phase of their flights to contact them.

In Queensland, two people suffered minor injuries on Saturday after a seaplane carrying five people crashed off the coast of Hamilton Island in the Whitsundays.

The occupants were picked up by tour boat operator the Ocean Explorer and taken to shore, where paramedics assessed them.

The fundraiser has so far raised almost A$25,000 of a A$40,000 target.

Emergency services were called to a boat ramp on Floyd Bay at Lansvale, in Sydney’s southwest, after the trio were seen in the water in distress shortly after 10am on Saturday.

NSW Police confirmed a 32-year-old woman was pulled from the water a short time later but she could not be revived.

The bodies of a 5-year-old girl and a 7-year-old boy were discovered about 2pm after an extensive search.

Fairfield Police Area Command Acting Superintendent Luke Scott said the woman was found with the help of bystanders and with the support of boats that were commandeered for the search efforts.

“With their assistance we were able to move swiftly through that search op but those two children were unfortunately found deceased,” he said.

“This investigation has moved from a search operation to … (an investigation) into how those people came into the water.

“They were on that pier, the circumstances of how they came off the pier and into the water is what we’re looking into.”

Scott said it’s understood the trio had been at the Shearer Park before they were spotted in the water.

He said that one scenario police were considering was whether the children had fallen into the water and the woman had jumped into rescue them.

However, he stressed the exact circumstances of the tragedy remain “unknown”.

“There’s a number of lines of inquiry we’re looking into … it’s very early in our investigation,” AScott said.

“The three of them were alone, there was nobody else there from their group.”

Scott thanked those members of the public who assisted in the search efforts.

“These are completely tragic circumstances,” he said.

“It’s beyond words for how tragic it will be for anyone who observed this and the family (involved).”

Police believe the current was very strong when the trio fell into the water, with rain, wind, and a “number of environmental factors” affecting the search efforts.

The search involved multiples agencies, including police, Marine Area Command, the Polair helicopter, and SES.

A report will be prepared for the information of the coroner regarding the three deaths.