The nation's population grew by an estimated 56,300 people in the year ended March, and now stands at 4.36 million, according to Statistics New Zealand.

The 1.3 per cent increase was the highest annual rate of growth since 2004, when the population increased by 1.6 per cent.

In the March 2009 year, the population grew by 1.0 per cent.

Natural increase, an excess of births over deaths, was the main reason for the increase over the past year, said population statistics manager Denise McGregor.

However, fewer permanent and long-term departures also resulted in an increased net migration, she said. As at March 31, half the male population was older than 35.5, and half the females were over 37.5.

Over the past 10 years, the median age has increased by 2.1 years for males and 2.6 years for females.

This particularly reflected population growth in the 45-69 year age groups, Mrs McGregor said.