Cardiac computed tomography angiography (CCTA) is a commonly used diagnostic tool for cardiovascular disease. Despite constant improvements to imaging technologies, the radiation dose to patients from CCTA remains a concern when using this procedure. There remains a need for optimisation of CCTA procedures and accurate dose monitoring to reduce the potential risk of cancer. Establishing diagnostic reference levels (DRLs) allows for the assessment of radiation dose variations, enabling strategies aimed at standardising doses across radiological centres. This systematic review explores the literature on CCTA methodologies that have been used to establish DRLs. A search was carried out using the Web of Science, SCOPUS, Medline, CINAHL and EMBASE databases. Reference lists of published articles were also assessed to identify further articles. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) methodology was employed to evaluate articles for relevance. Articles were included if they assessed DRLs in CCTA. The search resulted in 448 articles, of which, six were included after a thorough screening process. The literature demonstrates a wide dose variation in reported CCTA DRLs ranging from 671 to 1510 mGy cm in DLP. Where reported, CTDIvol DRLs ranged from 26 to 70 mGy. Differences were found in the methodologies used for establishing CCTA DRLs, including the sampling methodology used for identifying suitable patients and scanning protocols. This current review emphasises the need for an international standardisation for DRLs establishment methods, to provide a more comparable global measurement of dose variations across CT sites.