There is an increasing expectation that early career researchers (ECRs) be active in communicating their research to diverse audiences both within and beyond academia. However, with limited time and an academic environment that disproportionately values certain research outputs over others, ECRs may struggle to build their expertise and confidence in communicating research to a diverse array of audiences. Applying self-efficacy theory as a framework, this study investigates the first-hand experiences of ECRs in research communication, with a specific interest in their perceptions of their own communication skills, how these skills are developed, and how their self-beliefs impact their practices. Through semi-structured interviews with 30 ECRs in Australia and Japan, we found that hands-on engagement is vital for building self-efficacy, but that much more engagement and confidence was reported in communicating to academic audiences, such as through journal articles. While most ECRs expressed a desire to engage more with audiences outside of academia, not all were active, and many reported a lack of self-belief. Barriers identified include a lack of training, experience, opportunity to observe peers, encouragement, and feedback. Further, the potential for negative backlash from online and unknown public audiences is also a concern. We also note that for researchers who may not possess strong English language skills, interactive styles of communication may prove more challenging than more 'passive' forms. Our findings suggest that the focus of ECR communication skill development could be broadened, with less emphasis on specific types of outputs, and more emphasis on how to tailor communication to suit different audiences and channels. Providing an encouraging environment, opportunities to observe others, and opportunities to engage directly with support and feedback, may also be effective in building ECRs' self-efficacy.