Despite Australian horse owners being encouraged to vaccinate their horses against Hendra virus to reduce the risk of this potentially fatal virus to horses and humans, vaccine uptake has been slow. Discourse around the vaccine has been characterised by polarisation and dissenting voices. In this study we interviewed horse owners (N=15) and veterinarians (N=10), revealing how expert knowledge, disqualification of lay knowledge and inadequate handling of uncertainty impacted divisive discourse around Hendra virus. We assert that more inclusive, reflective and ultimately more effective risk communication practices will result if the legitimacy of diverse knowledge sources and the inevitability of uncertainty are acknowledged.