The intensity and polarisation of the second harmonic signal generated at an interface depends on the orientational order of the interfacial molecules. When this order is relatively weak the polarisation dependence becomes independent of the order. A general relationship between the components of the surface second-order susceptibility tensor is derived for weak orientational order that is independent of the form of the molecular hyperpolarisibility which can be used to determine the effective refractive index of the interfacial layer.