Light plane (file picture). (Source: istock.com)

Three men, two of them teenagers, have died after their light plane crashed into the ocean in front of horrified onlookers at a popular fishing spot.

Authorities said the single-engine Cessna 210 nosedived into the water while rolling, several hundred metres offshore in Long Bay, some 85km south of Adelaide, about 4.30pm on Friday.

The pilot, a 57-year-old man from Morphett Vale, and his two passengers, aged 18 and 19, were found dead.

The Australian Transport Safety Bureau is investigating the cause of the crash and has appealed for anyone with footage to come forward.

Acting Chief Commissioner Colin McNamara said viewing the video recordings of the accident would be crucial.

“In the footage, the aircraft has come down into water while rolling to the left in a very steep nose-down trajectory a couple of 100 metres offshore,” he told reporters on Saturday.

“Understanding the sequence of events leading up to the aircraft entering that trajectory will be of central importance.”

Investigators will also interview witnesses and involved parties and collect any recorded flight tracking data.

Any aircraft components of interest will be recovered to the bureau’s Canberra technical facilities for further examination.

Rough seas hampered efforts to retrieve the wreckage, with a police helicopter, firefighters, SES and private sea operators involved in the operation.

A resident of nearby Goolwa told the ABC Radio Adelaide the scene near the crash was “chaos”.

Police officer Senior Constable Rebecca Stokes described the crash as “tragic news for three families in South Australia today”.

The crash site is a remote beach but a popular fishing spot where the Murray River meets the Southern Ocean.

Police will prepare a report for the coroner while a preliminary ATSB report was expected to take about two months to complete, McNamara said.

Meanwhile, floodwaters are hampering attempts to reach the scene of a light plane crash near Normanton in Queensland’s Gulf Country.

Emergency services were called to the scene after the aircraft went down 800m west of Normanton airport about 8pm on Friday.

The pilot is believed to have been the only person on board but police and ambulance services were still trying to gain access to the crash site on Saturday.

The ATSB has been informed and will investigate the cause of the crash.