Cognitive theories of generalised anxiety disorder (GAD) posit that cognitive and behavioural factors maintain the disorder. This study examined whether avoidance and safety behaviours mediated the relationship between cognitive factors and GAD symptoms. We also examined the reverse mediation model; that is, whether cognitive factors mediated the relationship between maladaptive behaviours and GAD symptoms. Undergraduate psychology students (N = 125 and N = 292) completed the Worry Behaviours Inventory (a recently developed measure of maladaptive behaviours associated with GAD), in addition to measures of intolerance of uncertainty, cognitive avoidance, metacognitive beliefs, and symptoms of GAD and depression. Analyses supported the reliability and validity of the WBI. We consistently found that engagement in maladaptive behaviours significantly mediated the relationship between cognitive factors and symptoms of GAD. The reverse mediation model was also supported. Our results are consistent with the contention that cognitive and behavioural factors contribute to GAD symptom severity.