When we first noticed this Hobby, along with its mate, it was diving on and trying to chase two Black Kites away from a huge gum. Black Kites are considerably larger than Hobbies but with our appearance the Kites must have thought back up had arrived for the Hobbies and they retreated.
The Hobbies returned to another large gum next to the first tree and we discovered they had a nest there, hence their reluctance to have Kites anywhere near their tree.
Further investigation revealed that the Kites had a nest in the original tree where the confrontation had taken place.
Later still we observed one of the Black Kites with a dead chick in its talons (refer photos of Black Kites) which we assume is that of an Australian Hobby.
The Hobbies justifiable fears had proved well founded.