The primary atomisation of a liquid jet by a coaxial stream of high speed gas is analysed by means of Proper Orthogonal Decomposition (POD) for gas to liquid momentum ratios, MR, from 182 to 727 and Weber numbers, We, from 22 to 88. The continuous liquid core is visualised by the optical connectivity technique. The full spray in the near nozzle region is visualised using shadowgraphy. It is found that universal POD modes exist for the continuous liquid core and the near nozzle full spray across all considered flow conditions. The universal POD modes are related to physical structures of the flow. The complexity of the flow, as determined by the energy of the POD modes, is found to be constant for the liquid core across the examined range of flow MR. On the contrary, the complexity of the full spray is inversely proportional to the flow MR. Correlations are established between the spatial and temporal scales of primary atomisation. In addition, a novel method to extrapolate the spatial and temporal scales of the atomisation process beyond the limits of the current measurement resolution is described and demonstrated. Estimates are provided on the number of samples and the sampling rate that are required to fully resolve the flow to specific temporal and spatial scales.