Enroute the pilot encountered unforecast fog and low cloud and decided to carry out a precautionary landing. Rain had fallen on the surrounding countryside, causing the pilot to have doubts over the surface condition of paddocks in the area. Hence, an unopened, new section of the Hume highway was selected. Low runs were made over the area to check the surface condition, which appeared suitable. A section about 1200 metres long was selected. A normal landing was achieved and as the aircraft slowed down it unexpectedly entered a 30 centimetre deep track across the highway surface. The undetected track had been left in the surface to allow vehicles to cross the constructed area. As a result of entering this area the nose and right main gear legs collapsed. Significant Factors The following factors were considered relevant to the development of the accident: 1. The pilot encountered weather conditions precluding further flight in visual conditions and therefore necessitating a precautionary landing. 2. The area selected for the precautionary landing contained a hazard (a 30 centimetre deep track) which the pilot did not see during a precautionary search from the air prior to landing.