After school, he moved to Pretoria to join the academy. He played for them at Under-19 level in 2016, and the following year he was selected in the South Africa Under-20 squad for the 2017 World Rugby Under 20 Championship. He started all five matches at the tournament, helping his side to a 23–23 draw against France, a 38–14 victory over Georgia and a 72–14 win over Argentina (a match in which Grobbelaar scored two of South Africa's eleven tries) as South Africa topped Pool C to qualify for the semi-finals. A 22–24 defeat to England in the semi-finals ended their hopes of winning the competition, but they bounced back in the third-place play-off to beat France 37–15, with Grobbelaar scoring a further two tries.