Purpose: Previous research has shown the psychological effects of trauma are often under recognised and under treated, however there is a lack of research analysing this in the New Zealand context. This study aimed to look at under diagnosis and under treatment of PTSD amongst major trauma patients in New Zealand. Methodology: Participants were recruited from the NZ Major Trauma Registry (NZ MTR; Injury Severity Score ≥12). Eligible participants were mailed a questionnaire containing a series of self reported 5 point rating scales that assesses DSM-V symptoms of PTSD. Baseline characteristics and demographic data were obtained from the MTR. Results: Among 210 patients with major trauma (24.9% response rate), 38 (18.1%) were classified as having PTSD. Of these, 19 (50%) had not been assessed or formally diagnosed. 10 patients (26.3%), had received a formal diagnosis of PTSD. Of the 38 patients classified with PTSD, 9 (23.7%) were absolutely or quite certain they had no new mental illness since the trauma, 9 (23.7%) thought there was a roughly 50:50 chance they had a new mental illness, and 10 (26.3%) were very certain they had a new mental illness but had not been formally diagnosed. 18.4% (7) patients with PTSD received pharmacotherapy only; 10.5% talk therapy only; and 21% received both. 50% of the trauma patients with PTSD had not received any form of treatment. Conclusion: Many people who develop PTSD following trauma fail to receive appropriate assessment, diagnosis or treatment. Further work is needed to ensure adequate systems are in place to allow identification and treatment of patients who develop PTSD following a major trauma.