In Biggles in Australia, Daly Flats was a place owned by Boller, a member of von Stalhein's espionage ring in Australia. Boller had ostensibly taken the place in order to clear to plant peanuts. In actual fact it served as the headquarters of the spy ring. The place had a landing ground for aircraft and was also a cache of weapons and other equipment for use by members of the organisation.
According to Taffy Walsh, the Flats was "up at the top of the river", above where the river turned north towards Arnhem Land. This would suggest that it was somewhere near confluence with the Katherine River. This is consistent with what Cozens says later. When he, Ginger and Bertie flew up the Daly River and past the confluence with the Fregusson River, he said that Daly Flats was not far off.
Daly Flats is a fictional location.