The flight was planned from Launceston to Hobart via Clark. This requires that the aircraft track via the 149 degree radial from Launceston to Clark. From Clark, the inbound track to Hobart is 122 degrees. When the pilot requested a taxi clearance at Launceston, the air traffic controller responded with a taxi clearance and an airways clearance in the same transmission. The airways clearance was 'Hobart One 7000'. This is a standard instrument departure (SID) which involves tracking direct from Launceston to Hobart, not via Clark as planned. The clearance was not prefixed with the word 'amended' as it should have been, to reflect that the aircraft was being cleared on a route different than the one planned by the pilot. The pilot read the clearance back and a short time later departed. After departure, the pilot made the required departure report including advice that he was tracking via 'Hobart One'. Later, when the pilot called Hobart Control Tower, it was found that he had tracked via his planned route and not via the route cleared by air traffic control.