BACKGROUND: Emergency department (ED) crowding caused by access block is an increasing public health issue and has been associated with impaired health care delivery, negative patient outcomes and increased staff workload. AIM: To investigate the impact of opening a new ED on patient and healthcare service outcomes. METHOD: A 24 month time series analysis was employed using deterministically linked data from the ambulance service and three ED and hospital admission databases in Queensland, Australia. RESULTS: Total volume of ED presentations increased 18%, while local population growth increased by 3%. Healthcare service and patient outcomes at the two pre-existing hospitals did not improve. These outcomes included ambulance offload time: (Hospital A PRE: 10mins, POST: 10mins, p<0.001; Hospital B PRE: 10mins, POST: 15mins, p<0.001); ED LOS: (Hospital A PRE: 242mins, POST: 246mins, p<0.001; Hospital B PRE: 182mins, POST: 210mins, p<0.001); and access block: (Hospital A PRE: 41%, POST: 46%, p<0.001; Hospital B PRE: 23%, POST: 40%, p<0.001). Time series modelling indicated that the effect was worst at the hospital furthest away from the new ED. CONCLUSIONS: An additional ED within the region saw an increase in the total volume of presentations at a rate far greater than local population growth, suggesting it either provided an unmet need or a shifting of activity from one sector to another. Future studies should examine patient decision making regarding reasons for presenting to a new or pre-existing ED. There is an inherent need to take a 'whole of health service area' approach to solve crowding issues.