An investigation was under way after a man declared dead by Australian paramedics following a car crash was found to be alive around an hour later.

The driver was trapped upside down in the wreckage of a Porsche in a Melbourne suburb and was pronounced dead after being treated at the scene.

The Australian Broadcasting Corporation said after ambulances left the man remained in the car for up to an hour as police began their investigations.

It was only when State Emergency Service volunteers finally began the process of removing what they thought was a corpse that they discovered a "feeble pulse".

An ambulance was rushed back to the crash site and the 30-year-old was taken to hospital in a critical condition.

Ambulance Victoria said it was investigating the bungle with the two paramedics involved traumatised by their mistake.

"These are two very experienced paramedics but obviously this has been an error and we will work with them through that," said spokesman Simon Thomson.

He added that the driver had horrific facial injuries and paramedics were confronted with a challenging scene but would not comment on whether their error compromised his survival chances.