Background: Peripheral intravenous catheters (PIVCs) are associated with adverse events such as bloodstream infections; thus clinically appropriate practice is important. However, there is limited research on PIVC use in ambulance settings. This study investigated the incidence of paramedic-inserted PIVCs, unused PIVCs, and factors that influenced practice. Methods: Electronic patient care records for Western Australian ambulance service patients who attended between January 1 and December 31, 2020 were retrospectively reviewed. Patient, environmental, and paramedic characteristics were explored. Binominal logistical regression models were used to identify factors associated with PIVC insertion and unused PIVCs. Results: A total of 187,585 records were included; 20.3% had a PIVC inserted and 44% remained unused. Factors associated with PIVC insertion were gender, age, problem urgency, chief complaint, and operational region. Age, chief complaint, and paramedic years of experience were associated with unused PIVCs. Discussion: This study identified multiple modifiable factors for the unnecessary insertion of PIVCs, which may be addressed through better education and mentoring of paramedics supported by clearer clinical guidelines. Conclusions: This is, to our knowledge, the first Australian state-wide study to report unused paramedic-inserted PIVC rates. As 44% remained unused, clinical indication guidelines and intervention studies to reduce PIVC insertion are warranted.