The Boeing B767 aircraft was operating an RPT passenger service from Sydney direct to Perth. As the aircraft was rotated for lift-off the co-pilot, who was the handling pilot for the flight, found that the aircraft was very nose heavy. The crew contacted load control and requested a loading check. The check disclosed that 400 kg of cargo had been loaded into position 11, the most forward bay, and this load had not been entered into the loading data. The crew received revised stabiliser trim settings which were confirmed during the arrival into Perth. The investigation found that the load controller had established what load had to be loaded onto the aircraft and had made the relevant weight and balance computations. However the load controller had not locked out the computer field related to freight as is required by the relevant procedures. This allowed the aircraft loaders to load the additional cargo and make an entry into the computer. This entry did not become obvious to the load controller and, because the load controller thought there was no changes to the original computations, the crew were not advised to reconfigure the aircraft. The operator has instituted training procedures designed to improve discipline in the loading system.