The purpose of the flight was to conduct an aerial functional check of several water bores on the station property. The pilot advised that the engine cowl inspection panel for the oil filler/dipstick came open in flight prior to arrival at the 4th bore. Because of the severity of the wind buffetting to the open panel, the pilot decided to land the aircraft at the airstrip adjacent to 4th bore to close the panel. Before landing he carried out an aerial inspection of the airstrip and assessed it as being suitable with adequate length available. On the first landing attempt some cattle crossed the strip and a go-around was carried out. The second approach was conducted with power and full flap at 60 knots. The pilot advised that during the landing flare the aircraft continued to float along the runway. Once the aircraft was fully on the ground he opted to continue with the landing rather than attempt a go around. The pilot applied heavy braking, however he was unable to prevent the aircraft overunning the strip where the nose wheel entered a drainage ditch. The pilot and passenger were able to exit the aircraft safely. Post flight examination found that the aircraft's nosegear attachment bracket was broken and the firewall was buckled.