Du Plessis made his test debut with the 1971 Springboks touring party that went to Australia for a three test series. The test team included
Joggie Jansen,
Syd Nomis,
Frik du Preez,
Jan Ellis, and
Piet Greyling. Du Plessis played eighthman in all three tests, which the Springboks won 18–6 in Sydney, 14–6 in
Brisbane, and 19–11 in Sydney. Noted for being outspoken by Craven, Du Plessis was soon in trouble with South Africa's rugby administration for wearing denim trousers with his Springbok blazer. His liberal views, in which he was influenced by
Frederik van Zyl Slabbert, and his subsequent support for the
Progressive Federal Party, also ran afoul of the conservative politics of rugby administrators. Nor was Du Plessis a stranger to controversial incidents during his domestic and international career. In the 1975 Springbok trials prior to the French tour, Du Plessis punched Kleintjie Grobler, knocking out the man that a journalist later described as "pesky" and an "enforcer". Du Plessis and Grobler then played together on the national team against the Tricoleres that year. But probably the most notable incident involved
Naas Botha, whom Du Plessis injured with a bone-crunching late tackle in the 1977 Currie Cup semifinal between Province and
Northern Transvaal. The subsequent penalty kick by full-back
Pierre Edwards was successful, and Province lost the match 17–15. The Loftus Versfeld faithful was so incensed that Du Plessis had to be escorted off the field by the police. In 1974 the Springboks faced the touring
British Lions, who swept through South Africa without a loss. Winning 19 of their 20 matches on South African soil (plus one in
Namibia and another in
Rhodesia), the Lions drew only in the last test match, 13-all. Along the way they smashed the hapless South Western Districts 97–0 at the Van Riebeeck Ground, with
J. J. Williams scoring 6 tries, and
Alan Old converting 15 times. Du Plessis played in the first two tests (8th man and flank) and was dropped for the third. The series was noted for the selectors' use of 33 players in an attempt to stem the British tide – only three men played in all four tests, namely
Hannes Marais, Jan Ellis and right wing,
Chris Pope. and led the team on the
1980 South Africa rugby union tour of South America in October although an injury sustained in a Currie Cup match restricted him to just two matches, including only one of the internationals against the South American Jaguars. Du Plessis's sudden decision to stop playing rugby before the 1981 tour to New Zealand was motivated by a combination of mental fatigue and the death of his Western Province teammate,
full-back Chris Burger in a Currie Cup match. Playing under Du Plessis' leadership against Free State at Bloemfontein in 1980, 28-year-old Burger died after incurring a neck injury, sustained either while being tackled by two players or during the subsequent loose scrum. Craven tried to convince Du Plessis to continue playing, and remained convinced that if Du Plessis had made himself available for the tour, South Africa would have won the series. In all Du Plessis played in 32 matches for South Africa, and was on the winning side 18 times in 22 tests. Under his captaincy the Springboks won thirteen matches and lost only twice. ==International caps==