Fortunately, the man (49) and his girlfriend (42) were able to swim to safety, being able to escape through the side windows.
The pair were uninjured, and were able to use a mobile phone to call for help.
Police speculated that some form of 'vigorous movement' had rocked the vehicle backwards and forwards, and that the parking brake was not properly set, leading to its premature release.
Austrian fire rescue teams were quickly on the scene, and used a crane to retrieve the Mercedes from a depth of 20 metres. A speedy recovery was essential to prevent pollution of the lake waters.
The German couple found their ardour had been quickly dampened by the waters of the lake, and may be faced with an expensive insurance bill as investigators try to find the cause.
Austrians have ranked sex in a car number seven in a list of unusual locations for making love.