An elderly woman's body has been recovered from the water at a popular Perth fishing spot after a car plunged into the ocean in what police have described as a "tragic accident".

Key points: The elderly couple were both in the car when it drove into the water

The elderly couple were both in the car when it drove into the water The man was able to swim to safety but his wife was trapped inside

The man was able to swim to safety but his wife was trapped inside Police are investigating whether the driver or the car was at fault

Police said an 81-year-old man was driving the Mitsubishi Magna wagon when it entered the water at Fremantle port's South Mole just after 7:00pm on Monday.

He managed to swim to safety but his 75-year-old wife was trapped in the submerged car and could not be rescued.

Several police vehicles and Water Police were deployed to the scene.

The woman's body was retrieved by police divers a short time later.

The man is recovering in Fiona Stanley Hospital, but it is understood his injuries are not serious.

Woman could not undo seatbelt: witnesses

Witnesses Rhys Wells and Alana Newman said the car headed straight for the rocks after the man appeared to "put his foot down".

"Whether he's got scared, he's put his foot down again and it's gone straight into the water," Ms Newman said.

Mr Wells and several others went down on to the rocks to help.

An elderly woman's body was recovered from the water early on Tuesday morning. ( Facebook: Sandra Seman-Bourke )

"[The driver] managed to get his way out of the window, and someone came down with an extension cord and threw that down to him and tied it around him and got him up on to the rocks and out of the water," he said.

Mr Wells said the woman tried to undo her seatbelt to escape from the car, but was not able to unclip it.

"She was just sort of looking as if trying to say 'help me' sort of thing, but it was all to late and there was not much we could do," he said.

The pair said one man in the area was going to jump in to help the woman, but the car went under too quickly.

Mr Wells said the elderly man later began asking him where his wife was, but he did not know what to tell him.

Ms Newman said the incident would remain with them forever.

"I was just screaming and crying, just watching what had happened," she said.

"All I remember was jumping out of the car and just screaming, watching it go down.

"You see it in the movies, but never in real life and it will never leave our heads."

'It just looks like a tragic accident'

Major Crash officers are investigating exactly what caused the crash.

"It just looks like a tragic accident, and unfortunately it's going to impact on the family quite severely," Detective Senior Sergeant Steve Potter said.

Detective Senior Sergeant Steve Potter said the car was left fully submerged. ( ABC News: Frances Bell )

"We'll have to look at the car and see what has happened and we'll interview the driver when he's able to be interviewed and try to work out what's happened."

He said the investigation would be thorough.

"We want to find out if the car was functioning properly [and check] there wasn't any faults with it that might have contributed to the crash," he said.

"We're going to have to look at all the factors … all that environment around there, could that have contributed, and we also look at if there was any input from the driver."

Husband 'very fortunate', police say

The car was retrieved from the water late on Tuesday morning.

Detective Senior Sergeant Potter said it was unclear why the woman was unable to free herself from the vehicle.

"I suppose the first thing that comes to mind is her age, and it may have been very fortunate that her husband was able to get out of the car," he said.

"The car was fully submerged when Water Police got to it."

Police are asking anyone who witnessed the crash or saw the wagon beforehand to contact them.