We will present results of Faraday Rotation Measure (RM) studies of three supernova remnants using the Australia Telescope Compact Array at 1.4 GHz. We have used the latest techniques to extract a very accurate RM from a single frequency band, leading to results that are free from the $n\pi$ ambiguities associated with traditional multi-band RM determination. We discuss the implications of our results in terms of our understanding of the magneto-ionic properties of supernova remnant shells, the ISM and the region in which the two interact.