Four people have died after two planes collided mid-air before crashing into a paddock in Melbourne's north this morning.

Emergency services, including police and paramedics, were called to two separate crash scenes near Mangalore Airport about 11.30am on Wednesday, finding the aircraft a few kilometres from each other.

The crash site was about seven kilometres south from a flight training school at Mangalore, near Seymour.

The bodies of all four people, yet to be formally identified, were found in the wreckage.

It is understood one man aged in his 30s and another in his 40s were on one of the planes, a privately-owned Beechcraft Travel Air D95A operated by the Peninsula Aero Club at Tyabb.

A plane has crashed near Seymour with at least one person on board. (Nine)

The two men were qualified as instructors and were conducting a training flight, Peninsula Aero Club president Jack Vevers said.

"It's devastating, they are dear people to us, they're friends, they're colleagues. We're all as a community quite devastated by this news," he said.

The Civil Aviation Safety Authority said the other plane was a Piper Seminole twin engine registered to Moorabbin Aviation Services, also on a training flight.

Mitchell Local Area Commander Inspector Peter Koger said there were witnesses to the planes colliding and one of the aircraft coming down.

"One plane almost certainly crashed immediately and the other plane crashed about two kilometres north from here and both were extensively damaged prior to colliding with the ground," Inspector Koger said.

"We're not sure why both aircraft were on the same trajectory," he added.

"Unfortunately they collided mid-air."

The Piper-Seminole aircraft, operated by flying school Moorabbin Aviation Services with a trainee and instructor on board, had just taken off from Mangalore airfield and reached 1200 metres, when it collided with the Beechcraft Travel Air.

The second plane had a pilot and instructor on board from Peninsula Aero Club.

Workers witnessed the mid-air collision whilst doing electrical cabling in a paddock at a Defence Force secure site.

Aerial images of the scene taken from the 9News helicopter show debris from the two planes scattered across grassland.

A second plane has also crashed near the same site. (Nine)

Victoria Police Leading Senior Constable Kendra Jackson said the Civil Aviation Safety Authority (CASA) and the Australian Transport Safety Bureau (ATSB) had been notified.

"Police are currently investigating and will prepare a report for the Coroner," she said.

Both planes had clearance to be in the airspace, but whether they had radio contact prior to the collision is still unknown.

Investigators from the ATSB are set to comb through logbooks and maintenance records as part of the investigation.

In a statement, the ATSB said an investigation into the cause of the collision was underway.

"Transport safety investigators with experience in human factors, aircraft operations and maintenance from the ATSB's Melbourne, Canberra and Brisbane offices are preparing to deploy to the accident site," it read.

"On site, investigators will examine the wreckage and site surrounds.

"The ATSB will also analyse available recorded data, review weather information, and interview witnesses."

"The investigators that are deploying to the accident site have experience in human factors, aircraft operations and maintenance," ATSB Transport Safety executive director Nat Nagy said.

"Once they arrive on site the investigators will be looking at the aircrafts' wreckage as well as the site surrounds. The ATSB will also analyse available recorded data, review weather information, and interview witnesses."

Mangalore Airport is open to recreational flyers and those learning to fly, and is also used by state and federal government departments.

Mangalore Airport and Moorabbin Aviation Services declined to comment.