The occurrence of marine heatwaves (MHWs) around the World is increasing. The most severe MHW event in 140 years to occur in Australian waters happened during austral summer, 2011. Extreme ocean temperature events, such as this event, have disastrous effects on ocean ecosystems, causing mass mortality and habitat shifts. Ocean ecosystems are present at a range of depths, yet most MHW studies focus on surface waters due to a lack of subsurface in situ ocean measurements. Ocean temperature has been measured between the surface and 100 m depth at the Port Hacking national reference station (South of Sydney, Australia) since 1953. A daily climatology of temperature percentiles has been produced using measurements at this site, allowing the identification of MHW events and associated subsurface temperature extremes during a period of more than 60 years. We present the methodology for creating the climatology and highlight the challenges faced when combining big datasets collected over long timescales. We provide a consistent methodology with which other climatologies can be produced from similarly long combined datasets.