Extratropical cyclones that develop near the east coast of Australia often have severe consequences such as flash flooding and damaging winds and seas, as well as beneficial consequences such as being responsible for heavy rainfall events that contribute significantly to total rainfall and runoff. There is subjective evidence that the development of most major events, commonly known as East Coast Lows (ECLs), is associated with the movement of a high amplitude upper-tropospheric trough system over eastern Australia. To test this hypothesis, this study examines a number of large-scale diagnostic quantities in the upper-troposphere associated with extratropical cyclogenesis. Climatologies of these diagnostic quantities, based on reanalyses, are examined and compared with a database of observed ECL events. Results are examined in relation to seasonal and geographic variations. A diagnostic quantity based on geostrophic vorticity is shown to provide a good indication of the likely occurrence of east-coast cyclogenesis. The potential application of these diagnostics to global climate model simulations of past and future climates is also discussed.