Auckland is predicted to become home to almost 40% of the population and the Auckland Regional Council says a lot more work is needed to get it ready for the growth.
Latest figures from Statistics New Zealand project that the region will account for 60% of population growth by 2031.
The statistics predict just under two million people, or 38% of the population, will live in Auckland.
Auckland Regional Council says there's a risk the infrastructure to match that growth might not be ready.
Planning committee chairman Paul Walbren says he's confident the region's councils and the Government have planning on track and more will be done when the super-city is set up.
However, the leaders of some southern regions where population declines are forecast, say they have little faith in the predictions.
The projections suggest the West Coast's population will drop by 0.1% per year to just over 31,000 in 2031, and Southland's will also decrease, by 0.2% per annum, to just under 88,000 during the same time.
But regional leaders say the figures are based on historical trends and don't factor in Southland's potential to produce new energy and the value of its reliable rainfall.
They say West Coast coal mines and strong dairying and tourism in both regions, have also been ignored.