The purpose of this study is to assist in understanding the most effective parameters for TMS in Depressive Disorders. Most research conducted has used high frequency stimulation on the left hemisphere.However, low frequency on the right hemisphere has also been shown to have antidepressant properties and appears to be better tolerated. A promising approach to improve responses to rTMS may be to combine high and low frequency stimulation where they are both applied to the right side of the brain. In this approach, high frequency stimulation is provided first which may 'prime' or pre-prepare the brain for low frequency stimulation in a way that enhances its response. Participants are randomised to receive active or placebo priming stimulation.If participants do not respond to this treatment condition after 10 sessions applied over a two week period they will be offered 10 session of high frequency left sided TMS treatment. Alternatively, if participants respond favorably they may continue with that treatment condition for another 10 sessions.