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Background: Legionnaires' disease is a notifiable condition in New South Wales (NSW), Australia; clinicians and laboratories are required to report the disease to NSW Health. We describe the investigation of a sporadic case associated with the use of a communal spa pool in the case's apartment building complex and the use of whole genome sequencing to examine relatedness between clinical and environmental Legionella pneumophila serogroup 1 (Lp1) strains., Methods: In February 2018, a confirmed case of Lp1 infection was notified in a man in his 60s hospitalised with pneumonia. We asked the clinical team to obtain sputum in the event we found a potential source. The case described the use of the communal spa pool in his apartment building on two occasions during the putative exposure period. Environmental Health Officers from the Public Health Unit inspected the spa pool and found that the free chlorine level was well below the recommended concentration; a water sample was submitted for microbial analysis., Results: Lp1 was grown from the case's sputum and microbial analysis of the spa water sample found Lp1 at a concentration of 20 CFU/mL. The human and environmental isolates were subjected to whole genome sequencing and found to be highly genomically related. There was no other plausible environmental source of legionella., Conclusions: Whole genome sequencing of the clinical and environmental Lp1 isolates implicated a contaminated spa pool as the source of the case's exposure. This strongly supports the application of whole genome sequencing to the investigation of single cases of legionellosis. Communal spa pools in apartment buildings are not regulated in most Australian jurisdictions but must be considered to pose a potential legionella risk if improperly maintained., (© Commonwealth of Australia CC BY-NC-ND.)