**1. Introduction**

Few animal welfare issues have been as persistently contentious in Australia as the 'live export' of livestock via sea [1–3]. Recent media attention [4] has highlighted concerns regarding the effects of high environmental temperature and humidity on the welfare and mortality of animals being shipped live from Australia to the Middle East, especially during the Northern Hemisphere summer. These events have affected both sheep (*Ovis aries*) and cattle (*Bos taurus* and *B. indicus*), the species most commonly exported by Australia. Given the repeated occurrence of high heat load events with elevated sheep mortalities, and less common events involving cattle [5], how the industry is regulated [6–8] has been questioned [9–11]. A lack of understanding of what variables influence the likelihood of animals experiencing harmful heat load may have hindered attempts to predict and mitigate heat load events. Narrative reviews have been published on this issue in 2014 [11] and 2016 [12], and several unpublished reviews [13,14] have been produced since a media exposé in 2018 [2,4], but to our knowledge, no systematic literature reviews have previously been performed on this topic. Systematic reviews are particularly valuable in contentious situations in which bias on the part of authors may influence which studies are included and excluded (see Supplementary Material).
