SYDNEY - A Singaporean man on a tandem skydive in Australia and his instructor were killed after they crashed onto a driveway on a property outside Sydney.

Police are investigating the cause of the tragedy which occurred on Saturday (July 15) at about 2pm during a 4,200m-high dive operated by Sydney Skydivers.

The company said the instructor, who was in his 60s, was very experienced and the cause of the accident was unclear.

The Singaporean, who is said to be in his 20s and held an Australian work visa, is believed to have died on impact as did the instructor.

The young man's nationality was reported by local media but has yet to be officially confirmed.

The owner of Sydney Skydivers, Mr Phil Onis, said the instructor had been skydiving for 20 years. The firm claims to be the largest skydiving training centre in Australia.

"He was an experienced skydiver," Mr Onis told Sydney's Sunday Telegraph.

"We are keen to get in there and find out what happened. This has never happened before."

A police spokesman in the state of New South Wales would not confirm details or nationality of the victims.

"We will not be saying anything further until their next of kin have been identified," the spokesman told The Sunday Times.

The accident occurred at Wilton, a town about 85km south-west of Sydney.

The pair landed about a kilometre from their intended landing point on an open field.

Television footage taken by helicopter from above the driveway where the men landed appeared to show the parachutes, indicating they opened either partially or in full.

A Colombian tourist who took part in a separate jump, Ms Catalina Granados, said she caught a bus from Sydney to the skydiving base with the Singaporean. She said she was shocked by the tragedy, adding that the divers were told of the incident only when they returned to Sydney.

"I was in shock, I couldn't believe it," she told Daily Mail Australia. "The conditions on the flight were good and I never imagined this could happen."

A diver who went out before the Singaporean told Fairfax Media that instructors raised concerns about the wind direction but the jump was straightforward. Local wind speeds were reportedly relatively light at about 15 to 25 kmh.

Sydney Skydivers has reportedly been involved in at least four deaths since 2001. In 2012, a skydiver died after colliding with another diver in the air and then losing control of his parachute before crashing.

Police were notified about the deaths by a resident who saw the pair fall to the ground.

A small girl who lives on the property where the men fell saw the bodies on the driveway and is now receiving counselling.

"She has seen the aftermath," a co-owner of the property, Mr Chase Stephenson, told Sydney's Daily Telegraph. "She hasn't seen them hit the ground but she is pretty traumatised."

Officers are awaiting inquiries by aviation investigators and will prepare a report for the coroner.

The chief executive of the Australian Parachute Federation, Mr Brad Turner, said its investigators were assisting police.

"The main focus will be on the equipment as long as it's not too badly disturbed … we should be able to establish exactly what happened with the equipment," he told ABC News. "Whether it was equipment failure or human failure is something that will have to be established over time."