Objective This study aimed to examine trends in number of CT scans requested by tertiary emergency department (ED) physicians in Western Australia (WA) from 2003 to 2015 across broad demographic and presentation characteristics, anatomical areas and presented symptoms. Design An observational cross-sectional study over study period from 2003 to 2015. Setting Linked administrative health service data at individual level from WA. Participants A total of 1 666 884 tertiary hospital ED presentations of people aged 18 years or older were included in this study Main outcome measure Number of CT scans requested by tertiary ED physicians in an ED presentation. Methods Poisson regression models were used to assess variation and trends in number of CT scans requested by ED physicians across demographic characteristics, clinical presentation characteristics and anatomical areas. Results Over the entire study duration, 71 per 1000 ED episodes had a CT requested by tertiary ED physicians. Between 2003 and 2015, the rate of CT scanning almost doubled from 58 to 105 per 1000 ED presentations. After adjusted for all observed characteristics, the rate of CT scans showed a downward trend from 2009 to 2011 and subsequent increase. Males, older individuals, those attending ED as a result of pain, those with neurological symptoms or injury or with higher priority triage code were the most likely to have CT requested by tertiary ED physicians. Conclusions Noticeable changes in the number of CTs requested by tertiary ED physicians corresponded to the time frame of major health reforms happening within WA and nationally.