While on final approach to runway 36 at Darwin, the pilot observed a crane operating slightly east of the extended runway centreline. The pilot avoided the crane but its location and height concerned him. The pilot reported that he had not received notification about the crane from air traffic control. When the Civil Aviation Safety Authority (CASA) in Darwin was made aware of the crane's existence by Darwin ATC, the CASA District Aerodrome Inspector checked the information and found that the crane protruded 4.6 m through the Obstacle Limitation Surface (OLS) for the approach to runway 36. The crane operator had informed the aerodrome operator about the crane but the aerodrome safety officer who received the call had miscalculated the location and height of the crane, believing it to be just under the OLS. When the safety officer realised his mistake, he informed air traffic control. The crane has since been removed. LOCAL SAFETY ACTION CASA Darwin has advised that the District Aerodrome Inspector is reviewing the processes used by the aerodrome operator, from initial notification to the final output, to reduce the likelihood of a similar occurrence.