The Australian Nuclear Science and Technology Organisations (ANSTO) 1500L low-level radon monitor provides a unique technology for highly sensitive measurements of atmospheric radon (< 100 mBq m-3) and is recognised by the World Meteorological Organisation (WMO) as the best in the world for global and atmospheric compositional baseline studies. The ANSTO dual-flow-loop two-filter works by removing all ambient radon (222Rn) and thoron (220Rn) progeny in the sampled air from the airstream using a filter before the air passes into a large delay volume to allow for the decay of the short-lived isotope 220Rn. The sample air in the detector is passed through a flow loop at a flow rate designed to exchange the delay volume's air in approximately 20 minutes. This allows time for new radon progeny to form. A second flow loop circulates the volume of the delay chamber through the second filter (a low-impedance 625 mesh stainless steel screen) to collect all 218Po progeny. The newly formed progeny are collected on the second filter and their decays are counted with a ZnS-photomultiplier system. Using the flow rate and the count rate the atmospheric 222Rn concentrations can be calculated. This analyser has been used at Ridge Hill atmospheric monitoring station.