Mt Eden resident Alan Kemp is typical of many, having called the Unitary Plan a "rotten plan" based on bad numbers that allowed multi-storey buildings at odds with their surroundings.
The Employers and Manufacturers Association has also questioned the one million figure.
"The public of Auckland are being asked to pay additional infrastructure costs for a plan to accommodate a level of population that has a lower probability of being realised," says the association, which believes a more realistic projection was 600,000 to 750,000 people.
Speaking at a discussion on population growth trends, Mr Brown said Auckland's population grew by an average of 2.2 per cent between 1986 and 2011, above Statistics New Zealand's average high-growth rate of 1.8 per cent.
The latest population figure for Auckland was 1.5 million for the year to June 2012, an annual increase of 21,700 people or 1.46 per cent.
Said Mr Brown: "If we were to fall short, the consequences would be an undersupply of developable land in Auckland. If growth turns out to be slower, we will adjust our plans ..."
Former Government Statistician Len Cook was a guest speaker, saying Auckland's population attracted large numbers of people aged between 15 and 30, two-thirds of migrants and, at 10 per cent, had fewer people over 65 than the national average.
Asked if low family sizes among Europeans and Asians and reducing birth rates among Maori and Pacific Islanders could mean undershooting the one million figure, Mr Cook said there was a degree of risk with some upsides and some downsides.
Auckland growth
1,508,000 Latest population figure for Auckland
22,600 population increase per year over past five years
1.4 per cent population increase over the past year (medium growth)
2.4 per cent population increase between 2001 and 2006 (high growth)
Population by 2041
2.45m high projection
2.2m medium projection
1.97m low projection
Statistics New Zealand