The contribution of high order Zemike modes on average phase gradient is investigated. The resulting temporal power spectra are compared to data collected using the tip/tilt servo at the Sydney University Stellar Interferometer (SUSI). The low frequency part of the power spectrum follows a -2/3 power law which is in agreement with other similar work. Due to the effect of higher order terms, the high frequency power law for average phase gradient becomes -11/3 rather than the -17/3 predicted for the 'tilt' component of the Zemike representation of the wavefront.