Due to crossing traffic at flight level 180 the pilot of N103GC was asked if the aircraft could reach flight level 190 by 25 miles from Canberra. The pilot said this could not be done and air traffic control told the pilot to maintain flight level 170. During the climb the controller observed the mode C read out increase above flight level 170. The pilot was told to maintain flight level 170 but the level read out increased to flight level 176 before decreasing back to flight level 170. N103GC passed three to four miles behind the other aircraft, VH-TCH. Significant Factors The following factors were considered relevant to the development of the incident: 1. The pilot did not comply with the level restriction. 2. The precise reason for this happening was not determined.