The Australian Nuclear Science and Technology Organisation (ANSTO) maintains and disseminates the Australian standards of activity measurement. The standards include all nuclear medicine gamma-emitters and pure positron emitters. Calibration factors for the ANSTO 4 pi ionisation chamber for pure positron emitters have been traditionally determined from primary standardisations of cobalt-60 and sodium-22. Activity estimates of pure beta emitters have been previously determined by using 4 pi beta-gamma coincidence (efficiency tracer) counting. This method was adapted to test activity estimates of short-lived pure positron emitters made using the 4 pi ionisation chamber. Detailed are methods whereby the activity of fluorine-18 can be measured. The first method is an efficiency tracing method developed in this work. The method directly tests 4 pi ionisation chamber fluorine-18 activity estimates. The gamma-gamma method was carried out to confirm this.