Purnell began his rowing career at Shore School in Sydney. His senior club rowing has been from the Sydney University Boat Club. Purnell competed for the SUBC at the 2014 and 2015 Intervarsity Championships. In 2014 he rowed in the Sydney University eight and a coxed four and won both titles. In 2015 he rowed in the double scull and the eight and took the title in the eight. He first made state selection for New South Wales in the 2013 youth eight contesting the Noel Wilkinson Trophy at the Interstate Regatta within the Australian Rowing Championships. He again raced in the New South Wales youth eight in 2014 and 2015, rowing to an Interstate championship victory in 2015. In 2017 he was selected to stroke the New South Wales senior men's eight competing for the King's Cup at the Interstate Regatta. Purnell raced in three consecutive King's Cup victories for New South Wales from 2017 to 2019. He rowed in the seat behind his older brother Nicholas Purnell in those 2018 and 2019 victories. Nicholas is himself an Australian representative and Olympian rower.
International representative rowing
Alex Purnell made his Australian representative debut in 2013 in a junior quad scull who contested the 2013 Junior World Rowing Championships in Trakai to an eleventh place finish. In 2015 he was selected in an U23 quad scull which competed at the World Rowing U23 Championships in Plovdiv and rowed to a bronze medal. The following year at the 2016 World Rowing U23 Championships in Rotterdam he was seated in the Australian U23 eight which raced to a seventh place finish. 2017 saw Purnell elevated to the Australian senior squad and into the men's eight. He raced in the eight at two World Rowing Cups in Europe and then at the 2017 World Rowing Championships in Sarasota to a seventh placing. Purnell was selected in the Australian squad for their 2018 international tour. He rowed in the quad scull to a fourth place at the World Rowing Cup II in Linz, Austria. In the touring party's second regatta of the 2018 season Purnell was seated in the Australian men's eight racing as the Georgina Hope Rinehart National Training Centre, in honour of Rowing Australia patron, Gina Rinehart. 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.
