A woman has died after the car she was travelling in plunged into the water at the Bunbury port in Western Australia's South West.

Police are searching for a man they believe fled the area but said the crash appeared to be a "tragic accident".

Authorities said the alarm was raised about 4:50am on Saturday morning after the dark-coloured sedan crashed into the water and became "completely submerged".

The woman's body was found inside the car by water police divers.

The investigation into what caused the crash was continuing, police said.

"A man was seen in the water, however it is believed he left the area on foot," a police spokesman said.

"The search for that person is continuing.

"At this point in time it appears to be a tragic accident, however a formal conclusion on the cause of the crash will not be made until the man who was seen with the car is spoken to."

The man is described as being in his 30s, approximately 180–185 centimetres tall, of medium build and with shoulder length brown hair.

He was wearing jeans and a checked shirt.

Police said there were concerns the man may be injured, and urged him to come forward.

Anyone with information about the crash has been asked to call Crime Stoppers.