Abstract Background People who experience traumatic events have an increased risk of developing a range of mental disorders. Appropriate early support from a member of the public, whether a friend, family member, co-worker or volunteer, may help to prevent the onset of a mental disorder or may minimise its severity. However, few people have the knowledge and skills required to assist. Simple guidelines may help members of the public to offer appropriate support when it is needed. Methods Guidelines were developed using the Delphi method to reach consensus in a panel of experts. Experts recruited to the panels included 37 professionals writing, planning or working clinically in the trauma area, and 17 consumer or carer advocates who had been affected by traumatic events. As input for the panels to consider, statements about how to assist someone who has experienced a traumatic event were sourced through a systematic search of both professional and lay literature. These statements were used to develop separate questionnaires about possible ways to assist adults and to assist children, and panel members answered either one questionnaire or both, depending on experience and expertise. The guidelines were written using the items most consistently endorsed by the panels across the three Delphi rounds. Results There were 180 items relating to helping adults, of which 65 were accepted, and 155 items relating to helping children, of which 71 were accepted. These statements were used to develop the two sets of guidelines appended to this paper. Conclusions There are a number of actions which may be useful for members of the public when they encounter someone who has experienced a traumatic event, and it is possible that these actions may help prevent the development of some mental health problems in the future. Positive social support, a strong theme in these guidelines, has some evidence for effectiveness in developing mental health problems in people who have experienced traumatic events, but the degree to which it helps has not yet been adequately demonstrated. An evaluation of the effectiveness of these guidelines would be useful in determining their value. These guidelines may be useful to organisations who wish to develop or revise curricula of mental health first aid and trauma intervention training programs and policies. They may also be useful for members of the public who want immediate information about how to assist someone who has experienced a potentially traumatic event.