That eight won the 2018 Grand Challenge Cup at the Henley Royal Regatta - the fourth Australian men's eight to ever do so. He was back in the Australian quad for the 2018 World Rowing Championships in Plovdiv. That crew placed third in their heat and then in the repechage they went out hard and alongside New Zealand they surprised the Lithuanian world champions knocking them out of the final. In the final the Australian quad rowed through most of the field from the 1000m mark and finished in second place for a silver world championship medal. In 2019 he was selected with Caleb Antill, David Watts and Cameron Girdlestone to row Australia's quad scull for the 2019 international season. They placed 2nd at the World Rowing Cup II in Poznan and 4th at WRC III in Rotterdam. Purnell was then selected to race in the Australian men's eight at the 2019 World Rowing Championships in Linz, Austria. The eight were looking for a top five finish at the 2019 World Championships to qualify for the Tokyo Olympics. The eight placed second in their heat and fourth in the final and qualified for Tokyo 2020.