While the aircraft was descending to land at Canberra the forward toilet smoke detector activated, the flight deck smoke annunciate light illuminated and a small amount of smoke was visible on the flight deck. The flight attendant confirmed there was no evidence of fire in the toilet. A slight haze was visible in the cabin with wisps emanating from the airconditioning vents. The crew continued with the landing and a normal disembarkation ensued. Oil was found to be dripping from the engine which was removed for rectification. The number 1 air/oil seal was found to have delaminated allowing engine oil to permeate into the airconditioning ducts. The seal had previously been repaired at the manufacturers facility. After this incident the operator determined to use only newly manufactured seals and introduced an in-situ pressure test of the seal.