The condition of a student pilot from the Hong Kong Special Administrative Region (SAR) of China has deteriorated to critical on Friday after the aircraft he was training in crashed on the outskirts of Melbourne.

It took emergency services over one hour to free the young man and his instructor from the wreckage Thursday before they were rushed to separate hospitals with facial, chest and spinal injuries.

No detail has emerged as to what caused the BRM Aero Bristell-built Light Sport Aircraft to crash, however local media reported there was a thick fog on the day of the flight.

Recreational Aviation Australia (RAA), the body in charge of the investigation, told Xinhua that an investigation may take quite some time to complete.

"It will generally take about eight weeks to piece everything together, it's a fairly complex and involved process," RRA chief executive officer Michael Linke said.

"Over the course of next week we will learn a little bit more, but then we will need to test all of our theories and test all the data and that is the thing that takes time because the aircraft parts have to go and be tested and then we have to verify logbooks and find out a lot of information."

Although Royal Melbourne Hospital and the Alfred Hospital would not comment on the identity of the pair, RRA has released their surnames.

Cheung, from the Hong Kong SAR, is 19 years old and his instructor Dwyer is a 28-year-old pilot, with over "900 hours" of flying experience.

One witness who saw the aircraft come down told local media, "It sounded like a lack of power basically and before I could see anything it was below the tree line."

The aircraft was registered to a company at Moorabbin Airport called Learn to Fly Melbourne.