Heunis made his test debut for the Springboks during the 1981 tour of New Zealand as a replacement for Gysie Pienaar in the third test on 12 September 1981 at Eden Park in Auckland. His first start for the Springboks in a test match was against the USA on 20 September 1981 at the Owl Creek Polo ground in Glenville, New York. Heunis played 14 test matches for the Springboks, scoring 41 points. He played a further 10 tour matches for the Springboks in which he scored 31 points.