The bodies of two small children and a man believed to be their father have been retrieved from a car that Australian police believe was driven off a wharf deliberately.

The estate car sank in deep waters in Port Lincoln and quickly sank in 100 feet of water.

Witnesses reported seeing a car speeding down the wharf before it disappeared into the water, South Australia state police said.

Police divers found the bodies of the man and children inside the submerged car about six hours later.

"We are not treating it as an accidental driving-off-the-wharf," Detective Superintendent Paul Yeomans said.

Police have not released the victims' names.