In 2013, Du Preez was included in the South Africa Under-20 squad for the 2013 IRB Junior World Championship. He started the pool matches against the United States (kicking six conversions) and England (scoring two conversions and four penalties). He was an unused substitute in their final pool match against hosts France, but returned to the starting line-up for their semi-final match against Wales and came on as a substitute in the third-placed play-off against New Zealand.