I have a report on the first meeting at Seagrove and it starts with this paragraph.
On Sunday, November 10th [1946], Seagrove Aerodrome, about 30 miles from Auckland, was the scene of the first motor racing meeting in New Zealand to be held on a sealed course. The meeting was staged by the New Zealand Motor Racing Drivers' Association [soon to become the Auckland Car Club], who secured special permission to use the site from the Air Department. Events for midget, stock and sports cars, and also for motorcycles, were watched by almost 7000 spectators.
It's interesting that they were able to run such an event on a Sunday. Was there a law change to keep Sundays free of sporting events because I'm sure when I started going to motor races most if not all were on Saturdays? I recall the trouble the clubs had to get Sunday meetings at Pukekohe.