I think it can be seen in the picture on the right, which shows the Queen's visit to the Civic shopping precinct in Canberra.
Below is a video my parents made showing Canberra in the early 1970s.
My parents had an 8mm camera and I've been digitising some of these memories for a while now.
My uncle Peter writes:
My fondest memory of the car was a bunch of us being chauffeured by Andy, resplendent in chauffeur’s cap, to a restaurant in Bondi Junction. Passers by were fascinated as a seemingly endless stream of patrons disgorged from the car. The Humber had a pair of jump seats as well as the front and rear bench seats, I think there were about twelve of us (before the days of seat belts) on that occasion.
Yes, my understanding is Andy’s Humber was used in the Queen’s coronation tour before he bought it.