Background: Internet-based cognitive behavioural therapy (iCBT) for generalized anxiety disorder (GAD) has been shown to be effective when guided by a clinician. The present study sought to replicate this finding, and determine whether support from a technician is as effective as guidance from a clinician. Method: Randomized controlled non-inferiority trial comparing three groups: Clinician-assisted vs. technician-assisted vs. delayed treatment. Community-based volunteers applied to the VirtualClinic (www.virtualclinic.org.au) research program and 150 participants with GAD were randomized. Participants in the clinician- and technician-assisted groups received access to an iCBT program for GAD comprising six online lessons, weekly homework assignments, and weekly supportive contact over a treatment period of 10 weeks. Participants in the clinician-assisted group also received access to a moderated online discussion forum. The main outcome measures were the Penn State Worry Questionnaire (PSWQ) and the Generalized Anxiety Disorder-7 Item (GAD-7). Completion rates were high, and both treatment groups reduced scores on the PSWQ (p<0.001) and GAD-7 (p<0.001) compared to the delayed treatment group, but did not differ from each other. Within group effect sizes on the PSWQ were 1.16 and 1.07 for the clinician- and technician-assisted groups, respectively, and on the GAD-7 were 1.55 and 1.73, respectively. At 3 month follow-up participants in both treatment groups had sustained the gains made at post-treatment. Participants in the clinician-assisted group had made further gains on the PSWQ. Approximately 81 minutes of clinician time and 75 minutes of technician time were required per participant during the 10 week treatment program. Conclusions: Both clinician- and technician-assisted treatment resulted in large effect sizes and clinically significant improvements comparable to those associated with face-to-face treatment, while a delayed treatment/control group did not improve. These results provide support for large scale trials to determine the clinical effectiveness and acceptability of technician-assisted iCBT programs for GAD. This form of treatment has potential to increase the capacity of existing mental health services. Trial Registration: Australian New Zealand Clinical Trials Registry ACTRN12609000563268.