In 1956 he appeared for the Waratahs against the visiting Springboks and also played in both Tests. He made the 1957–58 Australia rugby union tour of Britain, Ireland and France and played in four of the five Tests. In 1958 he made his second tour of New Zealand for two test appearances including an unexpected victory in the 2nd Test in Christchurch. He made further Test appearances in 1959 (twice against the British Lions) and in 1962 (twice against Fiji). On the 1961 tour of South Africa captained by Ken Catch-pole, Thornett played in both tests and made his debut as Australian captain in a tour match against South-West Africa where a draw was achieved.