Joshua Simmonds made his junior international debut for the Australia U–21 side in 2015, at the Sultan of Johor Cup, where the team finished fifth.[10]
Following two years in the national development squad, Simmonds made his senior international debut for the Kookaburras in 2018 during a test match against Argentina in Darwin, Northern Territory.[11] He then went on the represent the team during the International Hockey Open.[12] In November 2018, Simmonds was named in the Kookaburras squad for the 2019 calendar year.[13]
Simmonds started 2019 in the Kookaburras team during the FIH Pro League in Melbourne, Victoria.[14] Simmonds continued to represent the team during the Pro League, culminating with a gold medal in Amstelveen, following a 3–2 win over Belgium.
Simmonds was selected in the Kookaburras Olympics squad for the Tokyo 2020 Olympics. The team reached the final for the first time since 2004 but couldn't achieve gold, beaten by Belgium in a shootout.[15]