The Narrabri Stellar Intensity Interferometer (NSII) is an astronomical interferometer located at the Australia Telescope Compact Array (ATCA) near Narrabri, New South Wales, Australia. It was constructed in the late 1970s and early 1980s and was the first interferometer to be used for astronomical observations. The NSII consists of two 25-metre antennas separated by a baseline of up to 200 metres. It is used to measure the intensity of stars and other celestial objects by combining their light from two different directions. The NSII has been used for a variety of research projects, including mapping out star formation regions in our Galaxy, measuring the size of stars, and studying the structure of active galactic nuclei.