Immigration is contributing to Auckland's rapidly increasing population.

New Zealand might have an aging population, but the biggest city is full of young people.

The latest figures from Statistics New Zealand shows the region growing by 2.6 percent each year, the fastest in the country.

Figures from Statistics New Zealand also show Auckland has the lowest median age in the country, with half the population under the age of 34.

Massey University's Dr Paul Spoonley said the rapid growth is because Auckland remains New Zealand's major destination for migrants.

He said young adults often leave regions for cities like Auckland for jobs or excitement, and the other factor driving the trend was the arrival of immigrants.

Every region in New Zealand grew in population, except the West Coast.

The Tasman region had the highest median age, with half the population 46 or older.