He said even allowing for large influxes of people after 4pm, and additional fans in buildings, boats, and other areas, fitting 150,000 or more people in the waterfront area was "impossible".
To reach the figure, Mr Cooper used the "Jacobs method" for crowd calculations, named after the journalist and academic Herbert Jacobs, who created a system for measuring anti-war Vietnam crowds.
Mr Cooper first counted the number of people per square metre on the waterfront. He found there were around 1.7 people per square metre.
Next he calculated the area of the first 850 metres of Quay St, the Viaduct, and lower Queen St. By multiplying the total area by the density he reached a maximum figure of 70,638 people.
"Based on the photos, I would say it is definitely less than 70,000, and maybe 50,000. It would be impossible to fit [the council's estimate] in the waterfront space." In December he calculated that 30,000 had attended the Santa Parade, despite its organisers reporting a figure of 250,000 to 300,000.
A report this week by Auckland Tourism, Events and Economic Development (Ateed) said about 200,000 people came into the CBD on opening night. Ateed said 50,000 to 80,000 crammed into bars and restaurants and 120,000 to 150,000 celebrated along the waterfront, including Queens Wharf.
Ateed communications manager Simon Roche said the organisation's figure was provided by police who were confident of their estimate.
Crowd numbers
Auckland Council/Police estimate:
* 120,000-150,000 in Rugby World Cup-designated areas (total of 200,000 in CBD)
Statistician estimate:
* 50,000-70,000 people (excluding Britomart and Wyndham Quarter).