Triage is the formal nursing assessment of all patients who present to an Emergency Department (ED). The National Triage Scale (NTS) is used in most Australian EDs. Triage decision making involves the allocation of every patients presenting to an ED to one of the five NTS categories. The NTS directly relates a triage category to illness or injury severity and need for emergency care. Triage nurses' decisions not only have the potential to impact on the health outcomes of ED patients, they are also used, in part, to evaluate ED performance and allocate components of ED funding. This study was a correlational study that used survey methods. Triage decisions were classified as 'expected triage', 'overtriage' or 'undertriage' decisions. Participant's qualifications were allocated to five categories: 'nil'; 'emergency nursing'; 'critical care nursing'; 'midwifery'; and 'tertiary' qualifications. There was no correlation between triage decisions and length of experience in emergency nursing or triage. 'Expected triage' decisions were more common when the predicted triage category was Category 3 (P < 0.001) and 'overtriage' decisions were less common when the predicted triage category was Category 2 (P < 0.0010). The frequency of 'undertriage' decisions decreased significantly when the predicted triage category was Category 3 (P < 0.001) or Category 4 (P < 0.001). There was no correlation between triage decisions and qualifications in the 'nil', 'emergency nursing' or 'critical care nursing' categories. A midwifery qualification demonstrated a positive correlation with 'expected triage' decisions (P = 0.048) and a negative correlation with 'undertriage' decisions (P = 0.012). There was also a positive correlation between a tertiary qualification and 'expected triage' decisions (P = 0.012).