FACTUAL INFORMATION The pilot of the Cessna 172 (C172) landed the aircraft on runway 31R and vacated the runway at the A2 taxiway intersection. However, the pilot became confused by the numerous taxiway options for his return to the parking area and entered runway 35R, using it as a taxiway in a northerly direction. Runway 35R was not in use at the time and was therefore available for taxiing. Whilst on runway 35R the pilot did not notice that his aircraft had entered runway 31R. He had not requested, nor had he received, a clearance from air traffic control to use runway 31R. He did not see any signage that indicated that he was approaching the active runway and did not notice the gable markers that were set back from the crossing point of the two runways. A Piper PA28 had just taken off from runway 31R and passed over the C172 at a height of about 50 ft. A second PA28 on final approach to runway 31R was sent around by air traffic control. SAFETY ACTION During the course of the investigation the Bureau of Air Safety Investigation issued the following recommendation relating to this occurrence: R980057 (issued 25 May 1998) "The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority review the adequacy of movement area guidance signs and markings, particularly at runway-to-runway intersections. The review should focus initially on Moorabbin Aerodrome and other aerodromes with multiple runways and complex taxiway systems."