The goal of this clinical trial is to compare the efficacy of a maximum output shock for cardioverting atrial fibrillation between two commonly used defibrillators in New Zealand . These machines have different maximum energy outputs, and to date no head-to-head comparison cardioverting atrial fibrillation between the two has been undertaken. The main question it aims to answer is whether either device is more likely to cardiovert patients referred for atrial fibrillation. Participants will be randomized to undergo cardioversion with one of two defibrillators at either 200J or 360J. Participants in each arm will undergo up to three shocks at the energy-level to which they have been randomized, using a standardized procedure. For participants randomized to the lower energy level who fail to return to normal rhythm after three shocks, they will be given a fourth shock at the higher energy level. All participants will then be asked to undertake a blood test the day following the cardioversion, and receive a follow up phone call. These are to ensure there is no difference in the safety of the procedure between the two energy levels. It is worth noting that these two components of the study (the blood test and phone call) are the only additional time commitment that is expected to be involved if you choose to participate in the study.