Silicon optical fibres are generating much interest as a means to directly integrate semiconductor functionality within the fibre architecture to provide a platform for compact all optical signal processing. For example, the high optical nonlinearity of the core material opens up the potential for these fibres to be used for signal regeneration in very short device lengths. In this paper we characterise the nonlinear optical properties of a step index hydrogenated amorphous silicon (a-Si:H) optical fibre and demonstrate its use for broadband self-phase modulation (SPM). The fibre was fabricated using a high pressure chemical fluid technique  to deposit the semiconductor into the central hole of a silica capillary. The resulting fibre had a core diameter of 6m and a length of 1.5cm, with the linear transmission losses measured to be as low as 1.7dB/cm at 1550nm. As the effective nonlinear interaction length is reduced by the material loss, the realization of low losses is a necessary requirement for the observation of these processes.